Sqlite raise error

(This page was last modified on 2003/07/20 01:16:48 UTC)

The SQLite library understands most of the standard SQL
language. But it does omit some features
while at the same time
adding a few features of its own. This document attempts to
describe percisely what parts of the SQL language SQLite does
and does not support. A list of keywords is
given at the end.

In all of the syntax diagrams that follow, literal text is shown in
bold blue. Non-terminal symbols are shown in italic red. Operators
that are part of the syntactic markup itself are shown in black roman.

This document is just an overview of the SQL syntax implemented
by SQLite. Many low-level productions are omitted. For detailed information
on the language that SQLite understands, refer to the source code and
the grammar file «parse.y».

SQLite implements the follow syntax:

• ATTACH DATABASE
• BEGIN TRANSACTION
• comment
• COMMIT TRANSACTION
• COPY
• CREATE INDEX
• CREATE TABLE
• CREATE TRIGGER
• CREATE VIEW
• DELETE
• DETACH DATABASE
• DROP INDEX
• DROP TABLE
• DROP TRIGGER
• DROP VIEW
• END TRANSACTION
• EXPLAIN
• expression
• INSERT
• ON CONFLICT clause
• PRAGMA
• REPLACE
• ROLLBACK TRANSACTION
• SELECT
• UPDATE
• VACUUM

Details on the implementation of each command are provided in
the sequel.

---

ATTACH DATABASE

sql-statement ::=

ATTACH [DATABASE] database-filename AS database-name

The ATTACH DATABASE statement adds a preexisting database
file to the current database connection. If the filename contains
punctuation characters it must be quoted. The names ‘main’ and
‘temp’ refer to the main database and the database used for
temporary tables. These cannot be detached. Attached databases
are removed using the DETACH DATABASE
statement.

You can read from and write to an attached database, but you cannot
alter the schema of an attached database. You can only CREATE and
DROP in the original database.

You cannot create a new table with the same name as a table in
an attached database, but you can attach a database which contains
tables whose names are duplicates of tables in the main database. It is
also permissible to attach the same database file multiple times.

Tables in an attached database can be referred to using the syntax
database-name.table-name. If an attached table doesn’t have
a duplicate table name in the main database, it doesn’t require a
database name prefix. When a database is attached, all of its
tables which don’t have duplicate names become the ‘default’ table
of that name. Any tables of that name attached afterwards require the table
prefix. If the ‘default’ table of a given name is detached, then
the last table of that name attached becomes the new default.

When there are attached databases, transactions are not atomic.
Transactions continue to be atomic within each individual
database file. But if your machine crashes in the middle
of a COMMIT where you have updated two or more database
files, some of those files might get the changes where others
might not.

There is a compile-time limit of 10 attached database files.

Executing a BEGIN TRANSACTION statement locks all database
files, so this feature cannot (currently) be used to increase
concurrancy.

---

BEGIN TRANSACTION

sql-statement ::=

BEGIN [TRANSACTION [name]] [ON CONFLICT conflict-algorithm]

sql-statement ::=

END [TRANSACTION [name]]

sql-statement ::=

COMMIT [TRANSACTION [name]]

sql-statement ::=

ROLLBACK [TRANSACTION [name]]

Beginning in version 2.0, SQLite supports transactions with
rollback and atomic commit. See ATTACH for
an exception when there are attached databases.

The optional transaction name is ignored. SQLite currently
doesn’t allow nested transactions. Attempting to start a new
transaction inside another is an error.

No changes can be made to the database except within a transaction.
Any command that changes the database (basically, any SQL command
other than SELECT) will automatically start a transaction if
one is not already in effect. Automatically started transactions
are committed at the conclusion of the command.

Transactions can be started manually using the BEGIN
command. Such transactions usually persist until the next
COMMIT or ROLLBACK command. But a transaction will also
ROLLBACK if the database is closed or if an error occurs
and the ROLLBACK conflict resolution algorithm is specified.
See the documention on the ON CONFLICT
clause for additional information about the ROLLBACK
conflict resolution algorithm.

The optional ON CONFLICT clause at the end of a BEGIN statement
can be used to changed the default conflict resolution algorithm.
The normal default is ABORT. If an alternative is specified by
the ON CONFLICT clause of a BEGIN, then that alternative is used
as the default for all commands within the transaction. The default
algorithm is overridden by ON CONFLICT clauses on individual
constraints within the CREATE TABLE or CREATE INDEX statements
and by the OR clauses on COPY, INSERT, and UPDATE commands.

---

comment

comment ::=	SQL-comment | C-comment
SQL-comment ::=	— single-line
C-comment ::=	/* multiple-lines [*/]

Comments aren’t SQL commands, but can occur in SQL queries. They are
treated as whitespace by the parser. They can begin anywhere whitespace
can be found, including inside expressions that span multiple lines.

SQL comments only extend to the end of the current line.

C comments can span any number of lines. If there is no terminating
delimiter, they extend to the end of the input. This is not treated as
an error. A new SQL statement can begin on a line after a multiline
comment ends. C comments can be embedded anywhere whitespace can occur,
including inside expressions, and in the middle of other SQL statements.
C comments do not nest. SQL comments inside a C comment will be ignored.

---

COPY

sql-statement ::=

COPY [ OR conflict-algorithm ] [database-name .] table-name FROM filename [ USING DELIMITERS delim ]

The COPY command is an extension used to load large amounts of
data into a table. It is modeled after a similar command found
in PostgreSQL. In fact, the SQLite COPY command is specifically
designed to be able to read the output of the PostgreSQL dump
utility pg_dump so that data can be easily transferred from
PostgreSQL into SQLite.

The table-name is the name of an existing table which is to
be filled with data. The filename is a string or identifier that
names a file from which data will be read. The filename can be
the STDIN to read data from standard input.

Each line of the input file is converted into a single record
in the table. Columns are separated by tabs. If a tab occurs as
data within a column, then that tab is preceded by a baskslash «»
character. A baskslash in the data appears as two backslashes in
a row. The optional USING DELIMITERS clause can specify a delimiter
other than tab.

If a column consists of the character «N», that column is filled
with the value NULL.

The optional conflict-clause allows the specification of an alternative
constraint conflict resolution algorithm to use for this one command.
See the section titled
ON CONFLICT for additional information.

When the input data source is STDIN, the input can be terminated
by a line that contains only a baskslash and a dot:
«.«.

---

CREATE INDEX

sql-statement ::=	CREATE [TEMP | TEMPORARY] [UNIQUE] INDEX index-name ON [database-name .] table-name ( column-name [, column-name]* ) [ ON CONFLICT conflict-algorithm ]
column-name ::=	name [ ASC | DESC ]

The CREATE INDEX command consists of the keywords «CREATE INDEX» followed
by the name of the new index, the keyword «ON», the name of a previously
created table that is to be indexed, and a parenthesized list of names of
columns in the table that are used for the index key.
Each column name can be followed by one of the «ASC» or «DESC» keywords
to indicate sort order, but the sort order is ignored in the current
implementation. Sorting is always done in ascending order.

There are no arbitrary limits on the number of indices that can be
attached to a single table, nor on the number of columns in an index.

If the UNIQUE keyword appears between CREATE and INDEX then duplicate
index entries are not allowed. Any attempt to insert a duplicate entry
will result in an error.

The optional conflict-clause allows the specification of an alternative
default constraint conflict resolution algorithm for this index.
This only makes sense if the UNIQUE keyword is used since otherwise
there are not constraints on the index. The default algorithm is
ABORT. If a COPY, INSERT, or UPDATE statement specifies a particular
conflict resolution algorithm, that algorithm is used in place of
the default algorithm specified here.
See the section titled
ON CONFLICT for additional information.

The exact text
of each CREATE INDEX statement is stored in the sqlite_master
or sqlite_temp_master table, depending on whether the table
being indexed is temporary. Everytime the database is opened,
all CREATE INDEX statements
are read from the sqlite_master table and used to regenerate
SQLite’s internal representation of the index layout.

Non-temporary indexes cannot be added on tables in attached
databases. They are removed with the DROP INDEX
command.

---

CREATE TABLE

sql-command ::=	CREATE [TEMP | TEMPORARY] TABLE table-name (     column-def [, column-def]*     [, constraint]* )
sql-command ::=	CREATE [TEMP | TEMPORARY] TABLE table-name AS select-statement
column-def ::=	name [type] [[CONSTRAINT name] column-constraint]*
type ::=	typename | typename ( number ) | typename ( number , number )
column-constraint ::=	NOT NULL [ conflict-clause ] | PRIMARY KEY [sort-order] [ conflict-clause ] | UNIQUE [ conflict-clause ] | CHECK ( expr ) [ conflict-clause ] | DEFAULT value
constraint ::=	PRIMARY KEY ( name [, name]* ) [ conflict-clause ]| UNIQUE ( name [, name]* ) [ conflict-clause ] | CHECK ( expr ) [ conflict-clause ]
conflict-clause ::=	ON CONFLICT conflict-algorithm

A CREATE TABLE statement is basically the keywords «CREATE TABLE»
followed by the name of a new table and a parenthesized list of column
definitions and constraints. The table name can be either an identifier
or a string. Tables names that begin with «sqlite_» are reserved
for use by the engine.

Each column definition is the name of the column followed by the
datatype for that column, then one or more optional column constraints.
SQLite is typeless.
The datatype for the column does not restrict what data may be put
in that column.
All information is stored as null-terminated strings.
The UNIQUE constraint causes an index to be created on the specified
columns. This index must contain unique keys.
The DEFAULT constraint
specifies a default value to use when doing an INSERT.

Specifying a PRIMARY KEY normally just creates a UNIQUE index
on the primary key. However, if primary key is on a single column
that has datatype INTEGER, then that column is used internally
as the actual key of the B-Tree for the table. This means that the column
may only hold unique integer values. (Except for this one case,
SQLite ignores the datatype specification of columns and allows
any kind of data to be put in a column regardless of its declared
datatype.) If a table does not have an INTEGER PRIMARY KEY column,
then the B-Tree key will be a automatically generated integer. The
B-Tree key for a row can always be accessed using one of the
special names «ROWID«, «OID«, or «_ROWID_«.
This is true regardless of whether or not there is an INTEGER
PRIMARY KEY.

If the «TEMP» or «TEMPORARY» keyword occurs in between «CREATE»
and «TABLE» then the table that is created is only visible to the
process that opened the database and is automatically deleted when
the database is closed. Any indices created on a temporary table
are also temporary. Temporary tables and indices are stored in a
separate file distinct from the main database file.

The optional conflict-clause following each constraint
allows the specification of an alternative default
constraint conflict resolution algorithm for that constraint.
The default is abort ABORT. Different constraints within the same
table may have different default conflict resolution algorithms.
If an COPY, INSERT, or UPDATE command specifies a different conflict
resolution algorithm, then that algorithm is used in place of the
default algorithm specified in the CREATE TABLE statement.
See the section titled
ON CONFLICT for additional information.

CHECK constraints are ignored in the current implementation.
Support for CHECK constraints may be added in the future. As of
version 2.3.0, NOT NULL, PRIMARY KEY, and UNIQUE constraints all
work.

There are no arbitrary limits on the number
of columns or on the number of constraints in a table.
The total amount of data in a single row is limited to about
1 megabytes. (This limit can be increased to 16MB by changing
a single #define in the source code and recompiling.)

The CREATE TABLE AS form defines the table to be
the result set of a query. The names of the table columns are
the names of the columns in the result.

The exact text
of each CREATE TABLE statement is stored in the sqlite_master
table. Everytime the database is opened, all CREATE TABLE statements
are read from the sqlite_master table and used to regenerate
SQLite’s internal representation of the table layout.
If the original command was a CREATE TABLE AS then then an equivalent
CREATE TABLE statement is synthesized and store in sqlite_master
in place of the original command.
The text of CREATE TEMPORARY TABLE statements are stored in the
sqlite_temp_master table.

Tables are removed using the DROP TABLE
statement. Non-temporary tables in an attached database cannot be
dropped.

---

CREATE TRIGGER

sql-statement ::=

CREATE [TEMP | TEMPORARY] TRIGGER trigger-name [ BEFORE | AFTER ] database-event ON [database-name .] table-name trigger-action

sql-statement ::=

CREATE [TEMP | TEMPORARY] TRIGGER trigger-name INSTEAD OF database-event ON [database-name .] view-name trigger-action

database-event ::=

DELETE | INSERT | UPDATE | UPDATE OF column-list

trigger-action ::=

[ FOR EACH ROW | FOR EACH STATEMENT ] [ WHEN expression ] BEGIN     trigger-step ; [ trigger-step ; ]* END

trigger-step ::=

update-statement | insert-statement | delete-statement | select-statement

The CREATE TRIGGER statement is used to add triggers to the
database schema. Triggers are database operations (the trigger-action)
that are automatically performed when a specified database event (the
database-event) occurs.

A trigger may be specified to fire whenever a DELETE, INSERT or UPDATE of a
particular database table occurs, or whenever an UPDATE of one or more
specified columns of a table are updated.

At this time SQLite supports only FOR EACH ROW triggers, not FOR EACH
STATEMENT triggers. Hence explicitly specifying FOR EACH ROW is optional. FOR
EACH ROW implies that the SQL statements specified as trigger-steps
may be executed (depending on the WHEN clause) for each database row being
inserted, updated or deleted by the statement causing the trigger to fire.

Both the WHEN clause and the trigger-steps may access elements of
the row being inserted, deleted or updated using references of the form
«NEW.column-name» and «OLD.column-name«, where
column-name is the name of a column from the table that the trigger
is associated with. OLD and NEW references may only be used in triggers on
trigger-events for which they are relevant, as follows:

INSERT	NEW references are valid
UPDATE	NEW and OLD references are valid
DELETE	OLD references are valid

If a WHEN clause is supplied, the SQL statements specified as trigger-steps are only executed for rows for which the WHEN clause is true. If no WHEN clause is supplied, the SQL statements are executed for all rows.

The specified trigger-time determines when the trigger-steps
will be executed relative to the insertion, modification or removal of the
associated row.

An ON CONFLICT clause may be specified as part of an UPDATE or INSERT
trigger-step. However if an ON CONFLICT clause is specified as part of
the statement causing the trigger to fire, then this conflict handling
policy is used instead.

Triggers are automatically dropped when the table that they are
associated with is dropped.

Triggers may be created on views, as well as ordinary tables, by specifying
INSTEAD OF in the CREATE TRIGGER statement. If one or more ON INSERT, ON DELETE
or ON UPDATE triggers are defined on a view, then it is not an error to execute
an INSERT, DELETE or UPDATE statement on the view, respectively. Thereafter,
executing an INSERT, DELETE or UPDATE on the view causes the associated
triggers to fire. The real tables underlying the view are not modified
(except possibly explicitly, by a trigger program).

Example:

Assuming that customer records are stored in the «customers» table, and
that order records are stored in the «orders» table, the following trigger
ensures that all associated orders are redirected when a customer changes
his or her address:

CREATE TRIGGER update_customer_address UPDATE OF address ON customers 
  BEGIN
    UPDATE orders SET address = new.address WHERE customer_name = old.name;
  END;

With this trigger installed, executing the statement:

UPDATE customers SET address = '1 Main St.' WHERE name = 'Jack Jones';

causes the following to be automatically executed:

UPDATE orders SET address = '1 Main St.' WHERE customer_name = 'Jack Jones';

Note that currently, triggers may behave oddly when created on tables
with INTEGER PRIMARY KEY fields. If a BEFORE trigger program modifies the
INTEGER PRIMARY KEY field of a row that will be subsequently updated by the
statement that causes the trigger to fire, then the update may not occur.
The workaround is to declare the table with a PRIMARY KEY column instead
of an INTEGER PRIMARY KEY column.

A special SQL function RAISE() may be used within a trigger-program, with the following syntax

raise-function ::=

RAISE ( ABORT, error-message ) | RAISE ( FAIL, error-message ) | RAISE ( ROLLBACK, error-message ) | RAISE ( IGNORE )

When one of the first three forms is called during trigger-program execution, the specified ON CONFLICT processing is performed (either ABORT, FAIL or
ROLLBACK) and the current query terminates. An error code of SQLITE_CONSTRAINT is returned to the user, along with the specified error message.

When RAISE(IGNORE) is called, the remainder of the current trigger program,
the statement that caused the trigger program to execute and any subsequent
trigger programs that would of been executed are abandoned. No database
changes are rolled back. If the statement that caused the trigger program
to execute is itself part of a trigger program, then that trigger program
resumes execution at the beginning of the next step.

Triggers are removed using the DROP TRIGGER
statement. Non-temporary triggers cannot be added on a table in an
attached database.

---

CREATE VIEW

sql-command ::=

CREATE [TEMP | TEMPORARY] VIEW view-name AS select-statement

The CREATE VIEW command assigns a name to a pre-packaged
SELECT
statement. Once the view is created, it can be used in the FROM clause
of another SELECT in place of a table name.

You cannot COPY, DELETE, INSERT or UPDATE a view. Views are read-only
in SQLite. However, in many cases you can use a
TRIGGER on the view to accomplish the same thing. Views are removed
with the DROP VIEW
command. Non-temporary views cannot be created on tables in an attached
database.

---

DELETE

sql-statement ::=

DELETE FROM [database-name .] table-name [WHERE expr]

The DELETE command is used to remove records from a table.
The command consists of the «DELETE FROM» keywords followed by
the name of the table from which records are to be removed.

Without a WHERE clause, all rows of the table are removed.
If a WHERE clause is supplied, then only those rows that match
the expression are removed.

---

DETACH DATABASE

sql-command ::=

DETACH [DATABASE] database-name

This statement detaches an additional database connection previously
attached using the ATTACH DATABASE statement. It
is possible to have the same database file attached multiple times using
different names, and detaching one connection to a file will leave the
others intact.

This statement will fail if SQLite is in the middle of a transaction.

---

DROP INDEX

sql-command ::=

DROP INDEX [database-name .] index-name

The DROP INDEX statement removes an index added with the
CREATE INDEX statement. The index named is completely removed from
the disk. The only way to recover the index is to reenter the
appropriate CREATE INDEX command. Non-temporary indexes on tables in
an attached database cannot be dropped.

---

DROP TABLE

sql-command ::=

DROP TABLE table-name

The DROP TABLE statement removes a table added with the CREATE TABLE statement. The name specified is the
table name. It is completely removed from the database schema and the
disk file. The table can not be recovered. All indices associated
with the table are also deleted. Non-temporary tables in an attached
database cannot be dropped.

---

DROP TRIGGER

sql-statement ::=

DROP TRIGGER [database-name .] trigger-name

The DROP TRIGGER statement removes a trigger created by the
CREATE TRIGGER statement. The trigger is
deleted from the database schema. Note that triggers are automatically
dropped when the associated table is dropped. Non-temporary triggers
cannot be dropped on attached tables.

---

DROP VIEW

sql-command ::=

DROP VIEW view-name

The DROP VIEW statement removes a view created by the CREATE VIEW statement. The name specified is the
view name. It is removed from the database schema, but no actual data
in the underlying base tables is modified. Non-temporary views in
attached databases cannot be dropped.

---

EXPLAIN

sql-statement ::=

EXPLAIN sql-statement

The EXPLAIN command modifier is a non-standard extension. The
idea comes from a similar command found in PostgreSQL, but the operation
is completely different.

If the EXPLAIN keyword appears before any other SQLite SQL command
then instead of actually executing the command, the SQLite library will
report back the sequence of virtual machine instructions it would have
used to execute the command had the EXPLAIN keyword not been present.
For additional information about virtual machine instructions see
the architecture description or the documentation
on available opcodes for the virtual machine.

---

expression

expr ::=

expr binary-op expr | expr like-op expr | unary-op expr | ( expr ) | column-name | table-name . column-name | database-name . table-name . column-name | literal-value | function-name ( expr-list | * ) | expr (+) | expr ISNULL | expr NOTNULL | expr [NOT] BETWEEN expr AND expr | expr [NOT] IN ( value-list ) | expr [NOT] IN ( select-statement ) | ( select-statement ) | CASE [expr] ( WHEN expr THEN expr )+ [ELSE expr] END

like-op ::=

LIKE | GLOB | NOT LIKE | NOT GLOB

This section is different from the others. Most other sections of
this document talks about a particular SQL command. This section does
not talk about a standalone command but about «expressions» which are
subcomponents of most other commands.

SQLite understands the following binary operators, in order from
highest to lowest precedence:

||
*    /    %
+    -
<<   >>   &    |
<    <=   >    >=
=    ==   !=   <>   IN
AND   
OR

Supported unary operaters are these:

-    +    !    ~

Any SQLite value can be used as part of an expression.
For arithmetic operations, integers are treated as integers.
Strings are first converted to real numbers using atof().
For comparison operators, numbers compare as numbers and strings
compare using the strcmp() function.
Note that there are two variations of the equals and not equals
operators. Equals can be either
= or ==.
The non-equals operator can be either
!= or <>.
The || operator is «concatenate» — it joins together
the two strings of its operands.
The operator % outputs the remainder of its left
operand modulo its right operand.

The LIKE operator does a wildcard comparision. The operand
to the right contains the wildcards.
A percent symbol % in the right operand
matches any sequence of zero or more characters on the left.
An underscore _ on the right
matches any single character on the left.
The LIKE operator is
not case sensitive and will match upper case characters on one
side against lower case characters on the other.
(A bug: SQLite only understands upper/lower case for 7-bit Latin
characters. Hence the LIKE operator is case sensitive for
8-bit iso8859 characters or UTF-8 characters. For example,
the expression ‘a’ LIKE ‘A’ is TRUE but
‘æ’ LIKE ‘Æ’ is FALSE.). The infix
LIKE operator is identical the user function
like(X,Y).

The GLOB operator is similar to LIKE but uses the Unix
file globbing syntax for its wildcards. Also, GLOB is case
sensitive, unlike LIKE. Both GLOB and LIKE may be preceded by
the NOT keyword to invert the sense of the test. The infix GLOB
operator is identical the user function
glob(X,Y).

A column name can be any of the names defined in the CREATE TABLE
statement or one of the following special identifiers: «ROWID«,
«OID«, or «_ROWID_«.
These special identifiers all describe the
unique random integer key (the «row key») associated with every
row of every table.
The special identifiers only refer to the row key if the CREATE TABLE
statement does not define a real column with the same name. Row keys
act like read-only columns. A row key can be used anywhere a regular
column can be used, except that you cannot change the value
of a row key in an UPDATE or INSERT statement.
«SELECT * …» does not return the row key.

SQLite supports a minimal Oracle8 outer join behavior. A column
expression of the form «column» or «table.column» can be followed by
the special «(+)» operator. If the table of the column expression
is the second or subsequent table in a join, then that table becomes
the left table in a LEFT OUTER JOIN. The expression that uses that
table becomes part of the ON clause for the join.
The exact Oracle8 behavior is not implemented, but it is possible to
construct queries that will work correctly for both SQLite and Oracle8.

SELECT statements can appear in expressions as either the
right-hand operand of the IN operator or as a scalar quantity.
In both cases, the SELECT should have only a single column in its
result. Compound SELECTs (connected with keywords like UNION or
EXCEPT) are allowed.
A SELECT in an expression is evaluated once before any other processing
is performed, so none of the expressions within the select itself can
refer to quantities in the containing expression.

When a SELECT is the right operand of the IN operator, the IN
operator returns TRUE if the result of the left operand is any of
the values generated by the select. The IN operator may be preceded
by the NOT keyword to invert the sense of the test.

When a SELECT appears within an expression but is not the right
operand of an IN operator, then the first row of the result of the
SELECT becomes the value used in the expression. If the SELECT yields
more than one result row, all rows after the first are ignored. If
the SELECT yeilds no rows, then the value of the SELECT is NULL.

Both simple and aggregate functions are supported. A simple
function can be used in any expression. Simple functions return
a result immediately based on their inputs. Aggregate functions
may only be used in a SELECT statement. Aggregate functions compute
their result across all rows of the result set.

The functions shown below are available by default. Additional
functions may be written in C and added to the database engine using
the sqlite_create_function()
API.

abs(X)	Return the absolute value of argument X.
coalesce(X,Y,…)	Return a copy of the first non-NULL argument. If all arguments are NULL then NULL is returned. There must be at least 2 arguments.
glob(X,Y)	This function is used to implement the «Y GLOB X» syntax of SQLite. The sqlite_create_function() interface can be used to override this function and thereby change the operation of the GLOB operator.
ifnull(X,Y)	Return a copy of the first non-NULL argument. If both arguments are NULL then NULL is returned. This behaves the same as coalesce() above.
last_insert_rowid()	Return the ROWID of the last row insert from this connection to the database. This is the same value that would be returned from the sqlite_last_insert_rowid() API function.
length(X)	Return the string length of X in characters. If SQLite is configured to support UTF-8, then the number of UTF-8 characters is returned, not the number of bytes.
like(X,Y)	This function is used to implement the «Y LIKE X» syntax of SQL. The sqlite_create_function() interface can be used to override this function and thereby change the operation of the LIKE operator.
lower(X)	Return a copy of string X will all characters converted to lower case. The C library tolower() routine is used for the conversion, which means that this function might not work correctly on UTF-8 characters.
max(X,Y,…)	Return the argument with the maximum value. Arguments may be strings in addition to numbers. The maximum value is determined by the usual sort order. Note that max() is a simple function when it has 2 or more arguments but converts to an aggregate function if given only a single argument.
min(X,Y,…)	Return the argument with the minimum value. Arguments may be strings in addition to numbers. The mminimum value is determined by the usual sort order. Note that min() is a simple function when it has 2 or more arguments but converts to an aggregate function if given only a single argument.
nullif(X,Y)	Return the first argument if the arguments are different, otherwise return NULL.
random(*)	Return a random integer between -2147483648 and +2147483647.
round(X) round(X,Y)	Round off the number X to Y digits to the right of the decimal point. If the Y argument is omitted, 0 is assumed.
soundex(X)	Compute the soundex encoding of the string X. The string «?000» is returned if the argument is NULL. This function is omitted from SQLite by default. It is only available the -DSQLITE_SOUNDEX=1 compiler option is used when SQLite is built.
sqlite_version(*)	Return the version string for the SQLite library that is running. Example: «2.8.0»
substr(X,Y,Z)	Return a substring of input string X that begins with the Y-th character and which is Z characters long. The left-most character of X is number 1. If Y is negative the the first character of the substring is found by counting from the right rather than the left. If SQLite is configured to support UTF-8, then characters indices refer to actual UTF-8 characters, not bytes.
typeof(X)	Return the type of the expression X. The only return values are «numeric» and «text». SQLite’s type handling is explained in Datatypes in SQLite.
upper(X)	Return a copy of input string X converted to all upper-case letters. The implementation of this function uses the C library routine toupper() which means it may not work correctly on UTF-8 strings.

The following aggregate functions are available by default. Additional
aggregate functions written in C may be added using the
sqlite_create_aggregate() API.

avg(X)	Return the average value of all X within a group.
count(X) count(*)	The first form return a count of the number of times that X is not NULL in a group. The second form (with no argument) returns the total number of rows in the group.
max(X)	Return the maximum value of all values in the group. The usual sort order is used to determine the maximum.
min(X)	Return the minimum value of all values in the group. The usual sort order is used to determine the minimum.
sum(X)	Return the numeric sum of all values in the group.

---

INSERT

sql-statement ::=

INSERT [OR conflict-algorithm] INTO [database-name .] table-name [(column-list)] VALUES(value-list) | INSERT [OR conflict-algorithm] INTO [database-name .] table-name [(column-list)] select-statement

The INSERT statement comes in two basic forms. The first form
(with the «VALUES» keyword) creates a single new row in an existing table.
If no column-list is specified then the number of values must
be the same as the number of columns in the table. If a column-list
is specified, then the number of values must match the number of
specified columns. Columns of the table that do not appear in the
column list are filled with the default value, or with NULL if not
default value is specified.

The second form of the INSERT statement takes it data from a
SELECT statement. The number of columns in the result of the
SELECT must exactly match the number of columns in the table if
no column list is specified, or it must match the number of columns
name in the column list. A new entry is made in the table
for every row of the SELECT result. The SELECT may be simple
or compound. If the SELECT statement has an ORDER BY clause,
the ORDER BY is ignored.

The optional conflict-clause allows the specification of an alternative
constraint conflict resolution algorithm to use during this one command.
See the section titled
ON CONFLICT for additional information.
For compatibility with MySQL, the parser allows the use of the
single keyword REPLACE as an alias for «INSERT OR REPLACE».

---

ON CONFLICT clause

conflict-clause ::=	ON CONFLICT conflict-algorithm
conflict-algorithm ::=	ROLLBACK | ABORT | FAIL | IGNORE | REPLACE

The ON CONFLICT clause is not a separate SQL command. It is a
non-standard clause that can appear in many other SQL commands.
It is given its own section in this document because it is not
part of standard SQL and therefore might not be familiar.

The syntax for the ON CONFLICT clause is as shown above for
the CREATE TABLE, CREATE INDEX, and BEGIN TRANSACTION commands.
For the COPY, INSERT, and UPDATE commands, the keywords
«ON CONFLICT» are replaced by «OR», to make the syntax seem more
natural. But the meaning of the clause is the same either way.

The ON CONFLICT clause specifies an algorithm used to resolve
constraint conflicts. There are five choices: ROLLBACK, ABORT,
FAIL, IGNORE, and REPLACE. The default algorithm is ABORT. This
is what they mean:

ROLLBACK

When a constraint violation occurs, an immediate ROLLBACK
occurs, thus ending the current transaction, and the command aborts
with a return code of SQLITE_CONSTRAINT. If no transaction is
active (other than the implied transaction that is created on every
command) then this algorithm works the same as ABORT.

ABORT

When a constraint violation occurs, the command backs out
any prior changes it might have made and aborts with a return code
of SQLITE_CONSTRAINT. But no ROLLBACK is executed so changes
from prior commands within the same transaction
are preserved. This is the default behavior.

FAIL

When a constraint violation occurs, the command aborts with a
return code SQLITE_CONSTRAINT. But any changes to the database that
the command made prior to encountering the constraint violation
are preserved and are not backed out. For example, if an UPDATE
statement encountered a constraint violation on the 100th row that
it attempts to update, then the first 99 row changes are preserved
but changes to rows 100 and beyond never occur.

IGNORE

When a constraint violation occurs, the one row that contains
the constraint violation is not inserted or changed. But the command
continues executing normally. Other rows before and after the row that
contained the constraint violation continue to be inserted or updated
normally. No error is returned.

REPLACE

When a UNIQUE constraint violation occurs, the pre-existing rows
that are causing the constraint violation are removed prior to inserting
or updating the current row. Thus the insert or update always occurs.
The command continues executing normally. No error is returned.
If a NOT NULL constraint violation occurs, the NULL value is replaced
by the default value for that column. If the column has no default
value, then the ABORT algorithm is used.

When this conflict resolution strategy deletes rows in order to
statisfy a constraint, it does not invoke delete triggers on those
rows. But that may change in a future release.

The conflict resolution algorithm can be specified in three places,
in order from lowest to highest precedence:

1. On individual constraints within a CREATE TABLE or CREATE INDEX
   statement.

2. On a BEGIN TRANSACTION command.

3. In the OR clause of a COPY, INSERT, or UPDATE command.

The algorithm specified in the OR clause of a COPY, INSERT, or UPDATE
overrides any algorithm specified on the BEGIN TRANSACTION command and
the algorithm specified on the BEGIN TRANSACTION command overrides the
algorithm specified in the a CREATE TABLE or CREATE INDEX.
If no algorithm is specified anywhere, the ABORT algorithm is used.

---

PRAGMA

sql-statement ::=

PRAGMA name [= value] | PRAGMA function(arg)

The PRAGMA command is used to modify the operation of the SQLite library.
The pragma command is experimental and specific pragma statements may be
removed or added in future releases of SQLite. Use this command
with caution.

The pragmas that take an integer value also accept
symbolic names. The strings «on«, «true«, and «yes»
are equivalent to 1. The strings «off«, «false«,
and «no» are equivalent to 0. These strings are case-
insensitive, and do not require quotes. An unrecognized string will be
treated as 1, and will not generate an error. When the value
is returned it is as an integer.

The current implementation supports the following pragmas:

• PRAGMA cache_size;
  
  PRAGMA cache_size = Number-of-pages;

  Query or change the maximum number of database disk pages that SQLite
  will hold in memory at once. Each page uses about 1.5K of memory.
  The default cache size is 2000. If you are doing UPDATEs or DELETEs
  that change many rows of a database and you do not mind if SQLite
  uses more memory, you can increase the cache size for a possible speed
  improvement.

  When you change the cache size using the cache_size pragma, the
  change only endures for the current session. The cache size reverts
  to the default value when the database is closed and reopened. Use
  the default_cache_size
  pragma to check the cache size permanently.

• PRAGMA count_changes = ON; (1)
  
  PRAGMA count_changes = OFF; (0)

  When on, the COUNT_CHANGES pragma causes the callback function to
  be invoked once for each DELETE, INSERT, or UPDATE operation. The
  argument is the number of rows that were changed.

  This pragma may be removed from future versions of SQLite.
  Consider using the sqlite_changes() API function instead.

• PRAGMA database_list;

  For each open database, invoke the callback function once with
  information about that database. Arguments include the index and
  the name the datbase was attached with. The first row will be for
  the main database. The second row will be for the database used to
  store temporary tables.

• PRAGMA default_cache_size;
  
  PRAGMA default_cache_size = Number-of-pages;

  Query or change the maximum number of database disk pages that SQLite
  will hold in memory at once. Each page uses 1K on disk and about 1.5K in memory.
  This pragma works like the cache_size
  pragma with the additional
  feature that it changes the cache size persistently. With this pragma,
  you can set the cache size once and that setting is retained and reused
  everytime you reopen the database.

• PRAGMA default_synchronous;
  
  PRAGMA default_synchronous = FULL; (2)
  
  PRAGMA default_synchronous = NORMAL; (1)
  
  PRAGMA default_synchronous = OFF; (0)

  Query or change the setting of the «synchronous» flag in
  the database. The first (query) form will return the setting as an
  integer. When synchronous is FULL (2), the SQLite database engine will
  pause at critical moments to make sure that data has actually been
  written to the disk surface before continuing. This ensures that if
  the operating system crashes or if there is a power failure, the database
  will be uncorrupted after rebooting. FULL synchronous is very
  safe, but it is also slow.
  When synchronous is NORMAL (1, the default), the SQLite database
  engine will still pause at the most critical moments, but less often
  than in FULL mode. There is a very small (though non-zero) chance that
  a power failure at just the wrong time could corrupt the database in
  NORMAL mode. But in practice, you are more likely to suffer
  a catastrophic disk failure or some other unrecoverable hardware
  fault. So NORMAL is the default mode.
  With synchronous OFF (0), SQLite continues without pausing
  as soon as it has handed data off to the operating system.
  If the application running SQLite crashes, the data will be safe, but
  the database might become corrupted if the operating system
  crashes or the computer loses power before that data has been written
  to the disk surface. On the other hand, some
  operations are as much as 50 or more times faster with synchronous OFF.

  This pragma changes the synchronous mode persistently. Once changed,
  the mode stays as set even if the database is closed and reopened. The
  synchronous pragma does the same
  thing but only applies the setting to the current session.

• PRAGMA default_temp_store;
  
  PRAGMA default_temp_store = DEFAULT; (0)
  
  PRAGMA default_temp_store = MEMORY; (2)
  
  PRAGMA default_temp_store = FILE; (1)

  Query or change the setting of the «temp_store» flag stored in
  the database. When temp_store is DEFAULT (0), the compile-time default
  is used for the temporary database. When temp_store is MEMORY (2), an
  in-memory database is used. When temp_store is FILE (1), a temporary
  database file on disk will be used. Note that it is possible for
  the library compile-time options to override this setting. Once
  the temporary database is in use, its location cannot be changed.

  This pragma changes the temp_store mode persistently. Once changed,
  the mode stays as set even if the database is closed and reopened. The
  temp_store pragma does the same
  thing but only applies the setting to the current session.

• PRAGMA empty_result_callbacks = ON; (1)
  
  PRAGMA empty_result_callbacks = OFF; (0)

  When on, the EMPTY_RESULT_CALLBACKS pragma causes the callback
  function to be invoked once for each query that has an empty result
  set. The third «argv» parameter to the callback is set to NULL
  because there is no data to report. But the second «argc» and
  fourth «columnNames» parameters are valid and can be used to
  determine the number and names of the columns that would have been in
  the result set had the set not been empty.

• PRAGMA full_column_names = ON; (1)
  
  PRAGMA full_column_names = OFF; (0)

  The column names reported in an SQLite callback are normally just
  the name of the column itself, except for joins when «TABLE.COLUMN»
  is used. But when full_column_names is turned on, column names are
  always reported as «TABLE.COLUMN» even for simple queries.

• PRAGMA index_info(index-name);

  For each column that the named index references, invoke the
  callback function
  once with information about that column, including the column name,
  and the column number.

• PRAGMA index_list(table-name);

  For each index on the named table, invoke the callback function
  once with information about that index. Arguments include the
  index name and a flag to indicate whether or not the index must be
  unique.

• PRAGMA integrity_check;

  The command does an integrity check of the entire database. It
  looks for out-of-order records, missing pages, malformed records, and
  corrupt indices.
  If any problems are found, then a single string is returned which is
  a description of all problems. If everything is in order, «ok» is
  returned.

• PRAGMA parser_trace = ON; (1)
  
  PRAGMA parser_trace = OFF; (0)

  Turn tracing of the SQL parser inside of the
  SQLite library on and off. This is used for debugging.
  This only works if the library is compiled without the NDEBUG macro.

• PRAGMA show_datatypes = ON; (1)
  
  PRAGMA show_datatypes = OFF; (0)

  When turned on, the SHOW_DATATYPES pragma causes extra entries containing
  the names of datatypes of columns to be
  appended to the 4th («columnNames») argument to sqlite_exec()
  callbacks. When
  turned off, the 4th argument to callbacks contains only the column names.
  The datatype for table columns is taken from the CREATE TABLE statement
  that defines the table. Columns with an unspecified datatype have a
  datatype of «NUMERIC» and the results of expression have a datatype of
  either «TEXT» or «NUMERIC» depending on the expression.
  The following chart illustrates the difference for the query
  «SELECT ‘xyzzy’, 5, NULL AS empty «:

  show_datatypes=OFF		show_datatypes=ON
  azCol[0] = «xyzzy»; azCol[1] = «5»; azCol[2] = «empty»; azCol[3] = 0;		azCol[0] = «xyzzy»; azCol[1] = «5»; azCol[2] = «empty»; azCol[3] = «TEXT»; azCol[4] = «NUMERIC»; azCol[5] = «TEXT»; azCol[6] = 0;

• PRAGMA synchronous;
  
  PRAGMA synchronous = FULL; (2)
  
  PRAGMA synchronous = NORMAL; (1)
  
  PRAGMA synchronous = OFF; (0)

  Query or change the setting of the «synchronous» flag affecting
  the database for the duration of the current database connection.
  The synchronous flag reverts to its default value when the database
  is closed and reopened. For additional information on the synchronous
  flag, see the description of the
  default_synchronous pragma.

• PRAGMA table_info(table-name);

  For each column in the named table, invoke the callback function
  once with information about that column, including the column name,
  data type, whether or not the column can be NULL, and the default
  value for the column.

• PRAGMA temp_store;
  
  PRAGMA temp_store = DEFAULT; (0)
  
  PRAGMA temp_store = MEMORY; (2)
  
  PRAGMA temp_store = FILE; (1)

  Query or change the setting of the «temp_store» flag affecting
  the database for the duration of the current database connection.
  The temp_store flag reverts to its default value when the database
  is closed and reopened. For additional information on the temp_store
  flag, see the description of the
  default_temp_store pragma. Note that it is possible for
  the library compile-time options to override this setting.

• PRAGMA vdbe_trace = ON; (1)
  
  PRAGMA vdbe_trace = OFF; (0)

  Turn tracing of the virtual database engine inside of the
  SQLite library on and off. This is used for debugging. See the
  VDBE documentation for more
  information.

No error message is generated if an unknown pragma is issued.
Unknown pragmas are ignored.

---

REPLACE

sql-statement ::=

REPLACE INTO [database-name .] table-name [( column-list )] VALUES ( value-list ) | REPLACE INTO [database-name .] table-name [( column-list )] select-statement

The REPLACE command is an alias for the «INSERT OR REPLACE» variant
of the INSERT command. This alias is provided for
compatibility with MySQL. See the
INSERT command documentation for additional
information.

---

SELECT

sql-statement ::=	SELECT [ALL | DISTINCT] result [FROM table-list] [WHERE expr] [GROUP BY expr-list] [HAVING expr] [compound-op select]* [ORDER BY sort-expr-list] [LIMIT integer [( OFFSET | , ) integer]]
result ::=	result-column [, result-column]*
result-column ::=	* | table-name . * | expr [ [AS] string ]
table-list ::=	table [join-op table join-args]*
table ::=	table-name [AS alias] | ( select ) [AS alias]
join-op ::=	, | [NATURAL] [LEFT | RIGHT | FULL] [OUTER | INNER | CROSS] JOIN
join-args ::=	[ON expr] [USING ( id-list )]
sort-expr-list ::=	expr [sort-order] [, expr [sort-order]]*
sort-order ::=	ASC | DESC
compound_op ::=	UNION | UNION ALL | INTERSECT | EXCEPT

The SELECT statement is used to query the database. The
result of a SELECT is zero or more rows of data where each row
has a fixed number of columns. The number of columns in the
result is specified by the expression list in between the
SELECT and FROM keywords. Any arbitrary expression can be used
as a result. If a result expression is
* then all columns of all tables are substituted
for that one expression. If the expression is the name of
a table followed by .* then the result is all columns
in that one table.

The DISTINCT keyword causes a subset of result rows to be returned,
in which each result row is different. NULL values are not treated as
distinct from eachother. The default behavior is that all result rows
be returned, which can be made explicit with the keyword ALL.

The query is executed against one or more tables specified after
the FROM keyword. If multiple tables names are separated by commas,
then the query is against the cross join of the various tables.
The full SQL-92 join syntax can also be used to specify joins.
A sub-query
in parentheses may be substituted for any table name in the FROM clause.
The entire FROM clause may be omitted, in which case the result is a
single row consisting of the values of the expression list.

The WHERE clause can be used to limit the number of rows over
which the query operates.

The GROUP BY clauses causes one or more rows of the result to
be combined into a single row of output. This is especially useful
when the result contains aggregate functions. The expressions in
the GROUP BY clause do not have to be expressions that
appear in the result. The HAVING clause is similar to WHERE except
that HAVING applies after grouping has occurred. The HAVING expression
may refer to values, even aggregate functions, that are not in the result.

The ORDER BY clause causes the output rows to be sorted.
The argument to ORDER BY is a list of expressions that are used as the
key for the sort. The expressions do not have to be part of the
result for a simple SELECT, but in a compound SELECT each sort
expression must exactly match one of the result columns. Each
sort expression may be optionally followed by ASC or DESC to specify
the sort order.

The LIMIT clause places an upper bound on the number of rows
returned in the result. A negative LIMIT indicates no upper bound.
The optional OFFSET following LIMIT specifies how many
rows to skip at the beginning of the result set.
In a compound query, the LIMIT clause may only appear on the
final SELECT statement.
The limit is applied to the entire query not
to the individual SELECT statement to which it is attached.

A compound SELECT is formed from two or more simple SELECTs connected
by one of the operators UNION, UNION ALL, INTERSECT, or EXCEPT. In
a compound SELECT, all the constituent SELECTs must specify the
same number of result columns. There may be only a single ORDER BY
clause at the end of the compound SELECT. The UNION and UNION ALL
operators combine the results of the SELECTs to the right and left into
a single big table. The difference is that in UNION all result rows
are distinct where in UNION ALL there may be duplicates.
The INTERSECT operator takes the intersection of the results of the
left and right SELECTs. EXCEPT takes the result of left SELECT after
removing the results of the right SELECT. When three are more SELECTs
are connected into a compound, they group from left to right.

---

UPDATE

sql-statement ::=	UPDATE [ OR conflict-algorithm ] [database-name .] table-name SET assignment [, assignment]* [WHERE expr]
assignment ::=	column-name = expr

The UPDATE statement is used to change the value of columns in
selected rows of a table. Each assignment in an UPDATE specifies
a column name to the left of the equals sign and an arbitrary expression
to the right. The expressions may use the values of other columns.
All expressions are evaluated before any assignments are made.
A WHERE clause can be used to restrict which rows are updated.

The optional conflict-clause allows the specification of an alternative
constraint conflict resolution algorithm to use during this one command.
See the section titled
ON CONFLICT for additional information.

---

VACUUM

sql-statement ::=

VACUUM [index-or-table-name]

The VACUUM command is an SQLite extension modelled after a similar
command found in PostgreSQL. If VACUUM is invoked with the name of a
table or index then it is suppose to clean up the named table or index.
In version 1.0 of SQLite, the VACUUM command would invoke
gdbm_reorganize() to clean up the backend database file.

VACUUM became a no-op when the GDBM backend was removed from
SQLITE in version 2.0.0.
VACUUM was reimplimented in version 2.8.1. It now cleans
the database by copying its contents to a temporary database file and
reloading the original database file from the copy. This will eliminate
free pages, align table data to be contiguous, and otherwise clean up
the database file structure. The index or table name argument is now
ignored.

This command will fail if there is an active transaction. This
command has no effect on an in-memory database.

---

SQLite keywords

The following keywords are used by SQLite. Most are either reserved
words in SQL-92 or were listed as potential reserved words. Those which
aren’t are shown in italics. Not all of these words are actually used
by SQLite. Keywords are not reserved in SQLite. Any keyword can be used
as an identifier for SQLite objects (columns, databases, indexes, tables,
triggers, views, …) but must generally be enclosed by brackets or
quotes to avoid confusing the parser. Keyword matching in SQLite is
case-insensitive.

Keywords can be used as identifiers in three ways:

‘keyword’	Interpreted as a literal string if it occurs in a legal string context, otherwise as an identifier.
«keyword»	Interpreted as an identifier if it matches a known identifier and occurs in a legal identifier context, otherwise as a string.
[keyword]	Always interpreted as an identifer. (This notation is used by MS Access and SQL Server.)

Fallback Keywords

These keywords can be used as identifiers for SQLite objects without
delimiters.

ABORT   
AFTER   
ASC   
ATTACH   
BEFORE   
BEGIN   
DEFERRED   
CASCADE   
CLUSTER   
CONFLICT   
COPY   
CROSS   
DATABASE   
DELIMITERS   
DESC   
DETACH   
EACH   
END   
EXPLAIN   
FAIL   
FOR   
FULL   
IGNORE   
IMMEDIATE   
INITIALLY   
INNER   
INSTEAD   
KEY   
LEFT   
MATCH   
NATURAL   
OF   
OFFSET   
OUTER   
PRAGMA   
RAISE   
REPLACE   
RESTRICT   
RIGHT   
ROW   
STATEMENT   
TEMP   
TEMPORARY   
TRIGGER   
VACUUM   
VIEW   

Normal keywords

These keywords can be used as identifiers for SQLite objects, but
must be enclosed in brackets or quotes for SQLite to recognize them as
an identifier.

ALL   
AND   
AS   
BETWEEN   
BY   
CASE   
CHECK   
COLLATE   
COMMIT   
CONSTRAINT   
CREATE   
DEFAULT   
DEFERRABLE   
DELETE   
DISTINCT   
DROP   
ELSE   
EXCEPT   
FOREIGN   
FROM   
GLOB   
GROUP   
HAVING   
IN   
INDEX   
INSERT   
INTERSECT   
INTO   
IS   
ISNULL   
JOIN   
LIKE   
LIMIT   
NOT   
NOTNULL   
NULL   
ON   
OR   
ORDER   
PRIMARY   
REFERENCES   
ROLLBACK   
SELECT   
SET   
TABLE   
THEN   
TRANSACTION   
UNION   
UNIQUE   
UPDATE   
USING   
VALUES   
WHEN   
WHERE   

Special words

The following are not keywords in SQLite, but are used as names of
system objects. They can be used as an identifier for a different
type of object.

_ROWID_   
MAIN   
OID   
ROWID   
SQLITE_MASTER   
SQLITE_TEMP_MASTER   

---

Back to the SQLite Home Page

Summary: this tutorial discusses SQLite trigger, which is a database object fired automatically when the data in a table is changed.

What is an SQLite trigger

An SQLite trigger is a named database object that is executed automatically when an INSERT, UPDATE or DELETE statement is issued against the associated table.

When do we need SQLite triggers

You often use triggers to enable sophisticated auditing. For example, you want to log the changes in the sensitive data such as salary and address whenever it changes.

In addition, you use triggers to enforce complex business rules centrally at the database level and prevent invalid transactions.

SQLite CREATE TRIGGER statement

To create a new trigger in SQLite, you use the CREATE TRIGGER statement as follows:

CREATE TRIGGER [IF NOT EXISTS] trigger_name 
   [BEFORE|AFTER|INSTEAD OF] [INSERT|UPDATE|DELETE] 
   ON table_name
   [WHEN condition]
BEGIN
 statements;
END;
Code language: SQL (Structured Query Language) (sql)

In this syntax:

• First,  specify the name of the trigger after the CREATE TRIGGER keywords.
• Next, determine when the trigger is fired such as BEFORE, AFTER, or INSTEAD OF. You can create BEFORE and AFTER triggers on a table. However, you can only create an INSTEAD OF trigger on a view.
• Then, specify the event that causes the trigger to be invoked such as INSERT, UPDATE, or DELETE.
• After that, indicate the table to which the trigger belongs.
• Finally, place the trigger logic in the BEGIN END block, which can be any valid SQL statements.

If you combine the time when the trigger is fired and the event that causes the trigger to be fired, you have a total of 9 possibilities:

• BEFORE INSERT
• AFTER INSERT
• BEFORE UPDATE
• AFTER UPDATE
• BEFORE DELETE
• AFTER DELETE
• INSTEAD OF INSERT
• INSTEAD OF DELETE
• INSTEAD OF UPDATE

Suppose you use a UPDATE statement to update 10 rows in a table, the trigger that associated with the table is fired 10 times. This trigger is called FOR EACH ROW trigger. If the trigger associated with the table is fired one time, we call this trigger a FOR EACH STATEMENT trigger.

As of version 3.9.2, SQLite only supports FOR EACH ROW triggers. It has not yet supported the FOR EACH STATEMENT triggers.

If you use a condition in the WHEN clause, the trigger is only invoked when the condition is true. In case you omit the WHEN clause, the trigger is executed for all rows.

Notice that if you drop a table, all associated triggers are also deleted. However, if the trigger references other tables, the trigger is not removed or changed if other tables are removed or updated.

For example, a trigger references to a table named people, you drop the people table or rename it, you need to manually change the definition of the trigger.

You can access the data of the row being inserted, deleted, or updated using the OLD and NEW references in the form: OLD.column_name and NEW.column_name.

the OLD and NEW references are available depending on the event that causes the trigger to be fired.

The following table illustrates the rules.:

Action	Reference
INSERT	NEW is available
UPDATE	Both NEW and OLD are available
DELETE	OLD is available

SQLite triggers examples

Let’s create a new table called leads to store all business leads of the company.

CREATE TABLE leads (
	id integer PRIMARY KEY,
	first_name text NOT NULL,
	last_name text NOT NULL,
	phone text NOT NULL,
	email text NOT NULL,
	source text NOT NULL
);
Code language: SQL (Structured Query Language) (sql)

1) SQLite BEFORE INSERT trigger example

Suppose you want to validate the email address before inserting a new lead into the leads table. In this case, you can use a BEFORE INSERT trigger.

First, create a BEFORE INSERT trigger as follows:

CREATE TRIGGER validate_email_before_insert_leads 
   BEFORE INSERT ON leads
BEGIN
   SELECT
      CASE
	WHEN NEW.email NOT LIKE '%_@__%.__%' THEN
   	  RAISE (ABORT,'Invalid email address')
       END;
END;
Code language: SQL (Structured Query Language) (sql)

We used the NEW reference to access the email column of the row that is being inserted.

To validate the email, we used the LIKE operator to determine whether the email is valid or not based on the email pattern. If the email is not valid, the RAISE function aborts the insert and issues an error message.

Second, insert a row with an invalid email into the leads table.

INSERT INTO leads (first_name,last_name,email,phone)
VALUES('John','Doe','jjj','4089009334');
Code language: SQL (Structured Query Language) (sql)

SQLite issued an error: “Invalid email address” and aborted the execution of the insert.

Third, insert a row with a valid email.

INSERT INTO leads (first_name, last_name, email, phone)
VALUES ('John', 'Doe', 'john.doe@sqlitetutorial.net', '4089009334');
Code language: SQL (Structured Query Language) (sql)

Because the email is valid, the insert statement executed successfully.

SELECT
	first_name,
	last_name,
	email,
	phone
FROM
	leads;
Code language: SQL (Structured Query Language) (sql)

2) SQLite AFTER UPDATE trigger example

The phones and emails of the leads are so important that you can’t afford to lose this information. For example, someone accidentally updates the email or phone to the wrong ones or even delete it.

To protect this valuable data, you use a trigger to log all changes which are made to the phone and email.

First, create a new table called lead_logs to store the historical data.

CREATE TABLE lead_logs (
	id INTEGER PRIMARY KEY,
	old_id int,
	new_id int,
	old_phone text,
	new_phone text,
	old_email text,
	new_email text,
	user_action text,
	created_at text
);
Code language: SQL (Structured Query Language) (sql)

Second, create an AFTER UPDATE trigger to log data to the lead_logs table whenever there is an update in the email or phone column.

CREATE TRIGGER log_contact_after_update 
   AFTER UPDATE ON leads
   WHEN old.phone <> new.phone
        OR old.email <> new.email
BEGIN
	INSERT INTO lead_logs (
		old_id,
		new_id,
		old_phone,
		new_phone,
		old_email,
		new_email,
		user_action,
		created_at
	)
VALUES
	(
		old.id,
		new.id,
		old.phone,
		new.phone,
		old.email,
		new.email,
		'UPDATE',
		DATETIME('NOW')
	) ;
END;
Code language: SQL (Structured Query Language) (sql)

You notice that in the condition in the WHEN clause specifies that the trigger is invoked only when there is a change in either email or phone column.

Third, update the last name of John from Doe to Smith.

UPDATE leads
SET 
   last_name = 'Smith'
WHERE
   id = 1;
Code language: SQL (Structured Query Language) (sql)

The trigger log_contact_after_update was not invoked because there was no change in email or phone.

Fourth, update both email and phone of John to the new ones.

UPDATE leads
SET 
   phone = '4089998888',
   email = 'john.smith@sqlitetutorial.net'
WHERE
   id = 1;
Code language: SQL (Structured Query Language) (sql)

If you check the log table, you will see there is a new entry there.

SELECT
   old_phone, 
   new_phone, 
   old_email, 
   new_email, 
   user_action
FROM
   lead_logs;
Code language: SQL (Structured Query Language) (sql)

You can develop the AFTER INSERT and AFTER DELETE triggers to log the data in the lead_logs table as an excercise.

SQLite DROP TRIGGER statement

To drop an existing trigger, you use the DROP TRIGGER statement as follows:

DROP TRIGGER [IF EXISTS] trigger_name;
Code language: SQL (Structured Query Language) (sql)

In this syntax:

• First, specify the name of the trigger that you want to drop after the DROP TRIGGER keywords.
• Second, use the IF EXISTS option to delete the trigger only if it exists.

Note that if you drop a table, SQLite will automatically drop all triggers associated with the table.

For example, to remove the validate_email_before_insert_leads trigger, you use the following statement:

DROP TRIGGER validate_email_before_insert_leads;
Code language: SQL (Structured Query Language) (sql)

In this tutorial, we have introduced you to SQLite triggers and show you how to create and drop triggers from the database.

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