Show mysql connection error

Never display connection errors manually!

MySQLi will generate a warning if it is unable to open the connection to MySQL. This warning tells you all you need to know including the error code, error message, and the place in the code where it happened. Checking for the error manually will not give you any more information.

If you can’t see the warning and your connection cannot be created, it might mean that your PHP is not configured to show them. In that case, you must check the error log file on your server. If you do not know where that is, use phpinfo() to get that information and search for error_log. It will tell you where the error log file is located.

If there is no warning in the error logs, it could mean that your PHP has error reporting silenced (either completely or just warnings). Check your PHP configuration.
In the production environment these settings should be maintained:

• error_reporting must be E_ALL
• log_errors must be On
• display_errors must be Off

In the development environment these settings should be maintained:

• error_reporting must be E_ALL
• log_errors must be On
• display_errors must be On

As you can see in the error message your database username and password have been revealed to the end user. This is sensitive information that you do not want to show anyone. In fact, a normal user would not understand this cryptic message. This is why display_errors must always be switched off in the production environment. Logging the errors on the server is safe.

Warnings vs. Exceptions

Warnings do not stop the script. If a warning is emitted the script will keep on executing until it encounters a fatal error. In most cases, you would want to throw an exception to stop the script. Do not use die/exit! If mysqli connection cannot be made, an exception should be thrown and if it is unhandled it will bubble up and stop the script with a fatal error. You can configure mysqli to throw exceptions automatically. This is invaluable because all mysqli functions can fail for many reasons and they will not inform you about any problems unless you check for errors manually every single one of them. Use the following line before opening the connection:

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli('localhost', 'my_user', 'my_password', 'my_db');

Do not catch the exceptions unless you really know what to do with them! One possible use case is described in How to connect properly using mysqli

Can mysqli_error() show any connection-related problems?

No. mysqli_error($conn) expects that the mysqli connection was successful. $conn must be a valid mysqli connection otherwise you would get this error message:

Warning: mysqli_error(): Couldn’t fetch mysqli in C:…

Neither $conn->error nor mysqli_error($conn) can display any connection-related errors!

Related: Should I manually check for errors when calling “mysqli_stmt_prepare”?

MySQL is a widely used open source relational database management system (RDMS) owned by Oracle. It has over the years been the default choice for web-based applications and still remains popular in comparison to other database engines.

Read Also: How to Install Latest MySQL on RHEL/CentOS and Fedora

MySQL was designed and optimized for web applications – it forms an integral part of major web-based applications such as Facebook, Twitter, Wikipedia, YouTube, and many others.

Is your site or web application powered by MySQL? In this detailed article, we will explain how to troubleshoot problems and common errors in MySQL database server. We will describe how to determine the causes of the problems and what to do to solve them.

1. Can’t Connect to Local MySQL Server

One of the common client to server connection errors in MySQL is “ERROR 2002 (HY000): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2)”.

This error indicates that there is no MySQL server (mysqld) running on the host system or that you have specified a wrong Unix socket file name or TCP/IP port when trying to connect to the server.

Ensure that the server is running by checking a process named mysqld on your database server host using the ps command and grep command together as shown.

$ ps xa | grep mysqld | grep -v mysqld

If the above commands show no output, then the database server isn’t running. Therefore the client can’t connect to it. To start the server, run the following systemctl command.

$ sudo systemctl start mysql        #Debian/Ubuntu
$ sudo systemctl start mysqld       #RHEL/CentOS/Fedora

To verify the MySQL service status, use the following command.

$ sudo systemctl status mysql       #Debian/Ubuntu
$ sudo systemctl status mysqld      #RHEL/CentOS/Fedora

From the output of the above command, the MySQL service has failed. In such a case, you can try to restart it and check its status once more.

$ sudo systemctl restart mysql
$ sudo systemctl status mysql

In addition, if the server is running as shown by the following command, but you still see the above error, you should also verify that the TCP/IP port is blocked by a firewall or any port blocking service.

$ ps xa | grep mysqld | grep -v mysqld

To find the port the server is listening on, use the netstat command as shown.

$ sudo netstat -tlpn | grep "mysql"

2. Can’t Connect to MySQL Server

Another commonly encountered connection error is “(2003) Can’t connect to MySQL server on ‘server’ (10061)”, which means that the network connection has been refused.

Here, start by checking that there is a MySQL server running on the system as shown above. Also ensure that the server has network connections enabled and that the network port you are using to connect is the one configured on the server.

Other common errors you are likely to encounter when you try to connect to the MySQL server are:

ERROR 2003: Can't connect to MySQL server on 'host_name' (111)
ERROR 2002: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (111)

These errors indicate that the server might be running, however, you are trying to connect using a TCP/IP port, named pipe, or Unix socket file different from the one on which the server is listening.

3. Access Denied Errors in MySQL

In MySQL, a user account is defined in terms of a username and the client host or hosts from which the user can connect to the server. In addition, an account may also have authentication credentials such as a password.

Although there are many different causes of “Access denied” errors, one of the common causes is relating to the MySQL accounts that the server permits client programs to use when connecting. It indicates that username specified in the connection does not have privileges to access the database.

MySQL allows the creation of accounts that enable client users to connect to the server and access data managed by the server. In this regard, if you encounter an access denied error, check if the user account is allowed to connect to the server via the client program you are using, and possibly the host from which the connection is coming from.

You can see what privileges a given account has by running the SHOW GRANTS command as shown.

> SHOW GRANTS FOR 'tecmint'@'localhost';

You can grant privileges to a particular user on specific database to the remote ip address using the following commands in the MySQL shell.

> grant all privileges on *.test_db to 'tecmint'@'192.168.0.100';
> flush privileges;

Furthermore, access denied errors can also result from problems with connecting to MySQL, refer to the previously explained errors.

4. Lost Connection to MySQL Server

You may encounter this error due to one of the following reasons: poor network connectivity, connection timeout or a problem with BLOB values that are larger than max_allowed_packet. In case of a network connection problem, ensure that you have a good network connection especially if you are accessing a remote database server.

If it is a connection timeout problem, particularly when MySQL is trying to use an initial connection to the server, increase the value of the connect_timeout parameter. But in case of BLOB values that are larger than max_allowed_packet, you need to set a higher value for the max_allowed_packet in your /etc/my.cnf configuration file under [mysqld] or [client] section as shown.

[mysqld]
connect_timeout=100
max_allowed_packet=500M

If the MySQL configuration file is not accessible for you, then you can set this value using the following command in the MySQL shell.

> SET GLOBAL connect_timeout=100;
> SET GLOBAL max_allowed_packet=524288000;

5. Too Many MySQL Connections

In case a MySQL client encounters the “too many connections” error, it means that all available connections are in use by other clients. The number of connections (default is 151) is controlled by the max_connections system variable; you can remedy the problem by increasing its value to permit more connections in your /etc/my.cnf configuration file.

[mysqld]
max_connections=1000

6. Out of Memory MySQL

In case you run a query using the MySQL client program and encounter the error in question, it means that MySQL does not have enough memory to store the entire query result.

The first step is to ensure that the query is correct, if it is, then do the following:

• if you are using MySQL client directly, start it with --quick switch, to disable cached results or
• if you are using the MyODBC driver, the configuration user interface (UI) has an advanced tab for flags. Check “Do not cache result“.

Another great tool is, MySQL Tuner – a useful script that will connect to a running MySQL server and gives suggestions for how it can be configured for higher performance.

$ sudo apt-get install mysqltuner     #Debian/Ubuntu
$ sudo yum install mysqltuner         #RHEL/CentOS/Fedora
$ mysqltuner

For MySQL optimization and performance tuning tips, read our article: 15 Useful MySQL/MariaDB Performance Tuning and Optimization Tips.

7. MySQL Keeps Crashing

If you encounter this problem, you should try to find out whether the problem is that the MySQL server dies or whether its the client with an issue. Note that many server crashes are caused by corrupted data files or index files.

You can check the server status to establish for how long it has been up and running.

$ sudo systemctl status mysql       #Debian/Ubuntu
$ sudo systemctl status mysqld      #RHEL/CentOS/Fedora

Alternatively, run the following mysqladmin command to find uptime of MySQL server.

$ sudo mysqladmin version -p 

Other solutions include but not limited to stopping the MySQL server and enabling debugging, then start the service again. You can try to make a test case that can be used to repeat the problem. In addition, open an additional terminal window and run the following command to display MySQL process statistics while you run your other queries:

$ sudo mysqladmin -i 5 status
OR
$ sudo mysqladmin -i 5 -r status 

The Bottom Line: Determining What Is Causing a Problem or an Error

Although we have looked at some common MySQL problems and errors and also provided ways to troubleshoot and solve them, the most important thing with diagnosing an error is understanding what it means (in terms of what is causing it).

So how can you determine this? The following points will guide you on how to ascertain what is exactly causing a problem:

1. The first and most important step is to look into the MySQL logs which are stored in the directory /var/log/mysql/. You can use command line utilities such as tail to read through the log files.
2. If MySQL service fails to start, check its status using systemctl or use the journetctl (with the -xe flag) command under systemd to examine the problem.
3. You can also examine system log file such as /var/log/messages or similar for reasons for your problem.
4. Try using tools such as Mytop, glances, top, ps, or htop to check which program is taking all CPU or is locking the machine or to inspect whether you are running out of memory, disk space, file descriptors, or some other important resource.
5. Assuming that problem is some runaway process, you can always try to kill it (using the pkill or kill utility) so that MySQL works normally.
6. Supposing that the mysqld server is causing problems, you can run the command: mysqladmin -u root ping or mysqladmin -u root processlist to get any response from it.
7. If the problem is with your client program while trying to connect to the MySQL server, check why it is not working fine, try to get any output from it for troubleshooting purposes.

You might also like to read these following MySQL related articles:

1. Learn MySQL / MariaDB for Beginners – Part 1
2. How to Monitor MySQL/MariaDB Databases using Netdata on CentOS 7
3. How to Transfer All MySQL Databases From Old to New Server
4. Mytop – A Useful Tool for Monitoring MySQL/MariaDB Performance in Linux
5. 12 MySQL/MariaDB Security Best Practices for Linux

For more information, consult the MySQL Reference manual concerning Problems and Common Errors, it comprehensively lists common problems and error messages that you may encounter while using MySQL, including the ones we have discussed above and more.

1. PHP MySQLi Error Functions
2. Conclusion

MySQLi is a PHP function used to access the MySQL database server. You can use this extension if you have MySQL version 4.1.13 or above.

There are various MySQLi functions that you can use to perform different functions in PHP. In this article, we will learn MySQLi error functions.

We will also see how and where to use code examples and observe the outputs. For this tutorial, we will use MySQL version 8.0.27 and PHP version 7.4.1.

PHP MySQLi Error Functions

In this tutorial, we will learn about the following PHP MySQLi error functions:

• mysqli_error()
• mysqli_errno()
• mysqli_error_list()
• mysqli_connect_error()
• mysqli_connect_errno()

All these functions can be used in object-oriented style and procedural style. Let’s understand both syntax using the mysqli_error() function.

Syntax of mysqli_error() Function in Object Oriented Style

Syntax of mysqli_error() Function in Procedural Style

string mysqli_error ( mysqli $link )

mysqli_error() Function in MySQL

This function is used to output the last error description for the most recent function call (if there is any). It is used when you want to know whether the SQL query has an error or not.

mysqli_error() returns the error description and empty string if there is no error. See the following example.

Example Code Using Procedural Style:

<?php
    $host = "localhost";
    $username = "root";
    $password = "";
    $database = "person";

    $connection = mysqli_connect($host, $username, $password, $database) 
    or die("Connection Failed"); 

    $sql = "SELECT * FROM teacher";
    $result = mysqli_query($connection, $sql);
    $error_message = mysqli_error($connection);

    if($error_message == ""){
        echo "No error related to SQL query.";
    }else{
        echo "Query Failed: ".$error_message;
    }
    mysqli_close($connection);
?>

The code given above tries to make the connection using $host, $username, $password, $database variables and save this connection into the $connection variable.

mysqli_error() function will take this connection variable $connection as a parameter and check if there is any error caused by the recent MySQLi function call which is mysqli_query($connection, $sql) here.

Output:

Now, change the table name in the SQL query from teacher to person and observe the output given below.

Output:

We, as a developer, can easily understand that there is no person table in the person database (this is what it means in the above error).

Keep the table name changed and replace the line $error_message = mysqli_error($connection); with $error_message = $connection->error; to practice and understand the object oriented style using MySQLi error function.

mysqli_errno() Function in MySQL

mysqli_errno() works the same as mysqli_error() does, but it will return the error code instead of the error description.

Write the following code to practice and understand. You may have noticed that we use a procedural style to practice this function.

<?php
     $host = "localhost";
     $username = "root";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed"); 

     $sql = "SELECT * FROM person";
     $result = mysqli_query($connection, $sql);
     $error_message = mysqli_errno($connection);

     if($error_message == ""){
     	echo "No error related to SQL query.";
     }else{
     	echo "Query Failed: ".$error_message;
     }
    mysqli_close($connection);
?>

The code given above will show the following output where you will see a number as an error code.

Output:

The question is, why do we use this function to show the numbers only? Because if you want to print a user-friendly error message (custom message), you can use this error code in if-else statements.

See the following code and its output below.

<?php
     $host = "localhost";
     $username = "root";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed"); 

     $sql = "SELECT * FROM person";
     $result = mysqli_query($connection, $sql);
     $error_message = mysqli_errno($connection);

     if($error_message == 1146){
     	echo "You are trying to read the data from a table which doesn't exist in your 			database "."'".$database."'";
     }
    mysqli_close($connection);
?>

Output:

mysqli_error_list() Function in MySQL

This function is very useful for knowing the error code, SQL state, and error description because this function returns an array containing all the necessary information.

Example Code:

<?php
     $host = "localhost";
     $username = "root";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed"); 

     $sql = "SELECT * FROM person";
     $result = mysqli_query($connection, $sql);
     print_r(mysqli_error_list($connection));
     mysqli_close($connection);
?>

Output:

mysqli_connect_error() Function in MySQL

mysqli_connect_error() returns the error description from the last connection if there is any. Although, the die() function also tell about the unsuccessful connection but mysqli_connect_error() returns the error that we can understand easily.

Write the following code first, see its output, and then we’ll compare it with the output produced by mysqli_connect_error().

<?php
     $host = "localhost";
     $username = "root";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed"); 

     $sql = "SELECT * FROM person";
     $result = mysqli_query($connection, $sql);
     $error_message = mysqli_error($connection);

     if($error_message != ""){
     	echo "Query Failed: ".$error_message;
      }
    mysqli_close($connection);
?>

Output:

See the output given above; you can see that the error we can understand is somewhere in the middle.

Imagine, if you have 2 or 3 errors, it would not be easy to find out. Now, use the mysqli_connect_error() and see the difference using the following code and output.

<?php
     $host = "localhost";
     $username = "newroot";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed: ".mysqli_connect_error()); 

     $sql = "SELECT * FROM teacher";
     $result = mysqli_query($connection, $sql);
     $error_message = mysqli_error($connection);

     if($error_message != ""){
     	echo "SQL Query Failed: ".$error_message;
    }
    mysqli_close($connection);
?>

Output:

The above output clearly says that there is no user named newroot, which does not allow you to access the database.

mysqli_connect_errno() Function in MySQL

This function behaves like mysqli_connect_error() but displays the error code rather than the error message. We can use this error code to write custom error messages.

Example Code:

<?php
     $host = "localhost";
     $username = "newroot";
     $password = "";
     $database = "person";

     $connection = mysqli_connect($host, $username, $password, $database) 
     or die("Connection Failed: ".mysqli_connect_errno()); 

     $sql = "SELECT * FROM teacher";
     $result = mysqli_query($connection, $sql);
     $error_message = mysqli_error($connection);

     if($error_message != ""){
	     echo "SQL Query Failed: ".$error_message;
     }
     mysqli_close($connection);
?>

Output:

Conclusion

Considering all the discussion and examples, we have concluded two main categories. The first category shows the errors about SQL queries and the other about database connections.

Depending on the project needs, we can print the error message or the error code in each category.

MySQL — система управления базами данных (СУБД) с открытым исходным кодом от компании Oracle. Она была разработана и оптимизирована специально для работы веб-приложений. MySQL является неотъемлемой частью таких веб-сервисов, как Facebook, Twitter, Wikipedia, YouTube и многих других.

Эта статья расскажет, как определять, с чем связаны частые ошибки на сервере MySQL, и устранять их.

Не удаётся подключиться к локальному серверу

Одной из распространённых ошибок подключения клиента к серверу является «ERROR 2002 (HY000): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2)».

Эта ошибка означает, что на хосте не запущен сервер MySQL (mysqld) или вы указали неправильное имя файла сокета Unix или порт TCP/IP при попытке подключения.

Убедитесь, что сервер работает. Проверьте процесс с именем mysqld на хосте сервера, используя команды ps или grep, как показано ниже.

$ ps xa | grep mysqld | grep -v mysqld

Если эти команды не показывают выходных данных, то сервер БД не работает. Поэтому клиент не может подключиться к нему. Чтобы запустить сервер, выполните команду systemctl.

$ sudo systemctl start mysql        #Debian/Ubuntu
$ sudo systemctl start mysqld       #RHEL/CentOS/Fedora

Чтобы проверить состояние службы MySQL, используйте следующую команду:

$ sudo systemctl status mysql       #Debian/Ubuntu
$ sudo systemctl status mysqld      #RHEL/CentOS/Fedora

Если в результате выполнения команды произошла ошибка службы MySQL, вы можете попробовать перезапустить службу и ещё раз проверить её состояние.

$ sudo systemctl restart mysql
$ sudo systemctl status mysql

Если сервер работает (как показано) и вы по-прежнему видите эту ошибку, вам следует проверить, не заблокирован ли порт TCP/IP брандмауэром или любой другой службой блокировки портов.

Для поиска порта, который прослушивается сервером, используйте команду netstat.

$ sudo netstat -tlpn | grep "mysql"

Ещё одна похожая и часто встречающаяся ошибка подключения — «(2003) Can’t connect to MySQL server on ‘server’ (10061)». Это означает, что в сетевом соединении было отказано.

Следует проверить, работает ли в системе сервер MySQL (смотрите выше) и на тот ли порт вы подключаетесь (как найти порт, можно посмотреть выше).

Похожие частые ошибки, с которыми вы можете столкнуться при попытке подключиться к серверу MySQL:

ERROR 2003: Cannot connect to MySQL server on 'host_name' (111)
ERROR 2002: Cannot connect to local MySQL server through socket '/tmp/mysql.sock' (111)

Ошибки запрета доступа в MySQL

В MySQL учётная запись (УЗ) определяется именем пользователя и клиентским хостом, с которого пользователь может подключиться. УЗ может также иметь данные для аутентификации (например, пароль).

Причин для запрета доступа может быть много. Одна из них связана с учётными записями MySQL, которые сервер разрешает использовать клиентским программам при подключении. Это означает, что имя пользователя, указанное в соединении, может не иметь прав доступа к базе данных.

В MySQL есть возможность создавать учётные записи, позволяющие пользователям клиентских программ подключаться к серверу и получать доступ к данным. Поэтому при ошибке доступа проверьте разрешение УЗ на подключение к серверу через клиентскую программу.

Увидеть разрешённые привилегии учётной записи можно, выполнив в консоли команду SHOW GRANTS
Входим в консоль (пример для Unix, для Windows консоль можно найти в стартовом меню):

В консоли вводим команду:

> SHOW GRANTS FOR 'tecmint'@'localhost';

Дать привилегии конкретному пользователю в БД по IP-адресу можно, используя следующие команды:

> grant all privileges on *.test_db to 'tecmint'@'192.168.0.100';
> flush privileges;

Ошибки запрещённого доступа могут также возникнуть из-за проблем с подключением к MySQL (см. выше).

Потеря соединения с сервером MySQL

С этой ошибкой можно столкнуться по одной из следующих причин:

• плохое сетевое соединение;
• истекло время ожидания соединения;
• размер BLOB  больше, чем max_allowed_packet.

В первом случае убедитесь, что у вас стабильное сетевое подключение (особенно, если подключаетесь удалённо).

Если проблема с тайм-аутом соединения (особенно при первоначальном соединении MySQL с сервером), увеличьте значение параметра connect_timeout.

В случае с размером BLOB нужно установить более высокое значение для max_allowed_packet в файле конфигурации /etc/my.cnf в разделах [mysqld] или [client] как показано ниже.

[mysqld]
connect_timeout=100
max_allowed_packet=500M

Если файл конфигурации недоступен, это значение можно установить с помощью следующей команды.

> SET GLOBAL connect_timeout=100;
> SET GLOBAL max_allowed_packet=524288000;

Слишком много подключений

Эта ошибка означает, что все доступные соединения используются клиентскими программами. Количество соединений (по умолчанию 151) контролируется системной переменной max_connections. Устранить проблему можно, увеличив значение переменной в файле конфигурации /etc/my.cnf.

[mysqld]
max_connections=1000

Недостаточно памяти

Если такая ошибка возникла, это может означать, что в MySQL недостаточно памяти для хранения всего результата запроса.

Сначала нужно убедиться, что запрос правильный. Если это так, то нужно выполнить одно из следующих действий:

• если клиент MySQL используется напрямую, запустите его с ключом --quick switch, чтобы отключить кешированные результаты;
• если вы используете драйвер MyODBC, пользовательский интерфейс (UI) имеет расширенную вкладку с опциями. Отметьте галочкой «Do not cache result» (не кешировать результат).

Также может помочь MySQL Tuner. Это полезный скрипт, который подключается к работающему серверу MySQL и даёт рекомендации по настройке для более высокой производительности.

$ sudo apt-get install mysqltuner     #Debian/Ubuntu
$ sudo yum install mysqltuner         #RHEL/CentOS/Fedora
$ mysqltuner

MySQL продолжает «падать»

Если такая проблема возникает, необходимо выяснить, заключается она в сервере или в клиенте. Обратите внимание, что многие сбои сервера вызваны повреждёнными файлами данных или индексными файлами.

Вы можете проверить состояние сервера, чтобы определить, как долго он работал.

$ sudo systemctl status mysql       #Debian/Ubuntu
$ sudo systemctl status mysqld      #RHEL/CentOS/Fedora

Чтобы узнать время безотказной работы сервера, запустите команду mysqladmin.

$ sudo mysqladmin version -p 

Кроме того, можно остановить сервер, сделать отладку MySQL и снова запустить службу. Для отображения статистики процессов MySQL во время выполнения других процессов откройте окно командной строки и введите следующее:

$ sudo mysqladmin -i 5 status

Или

$ sudo mysqladmin -i 5 -r status

Заключение

Самое важное при диагностике — понять, что именно вызвало ошибку. Следующие шаги помогут вам в этом:

• Первый и самый важный шаг — просмотреть журналы MySQL, которые хранятся в каталоге /var/log/mysql/. Вы можете использовать утилиты командной строки вроде tail для чтения файлов журнала.
• Если служба MySQL не запускается, проверьте её состояние с помощью systemctl. Или используйте команду journalctl (с флагом -xe) в systemd.
• Вы также можете проверить файл системного журнала (например, /var/log/messages) на предмет обнаружения ошибок.
• Попробуйте использовать такие инструменты, как Mytop, glances, top, ps или htop, чтобы проверить, какая программа использует весь ресурс процессора или блокирует машину. Они также помогут определить нехватку памяти, дискового пространства, файловых дескрипторов или какого-либо другого важного ресурса.
• Если проблема в каком-либо процессе, можно попытаться его принудительно остановить, а затем запустить (при необходимости).
• Если вы уверены, что проблемы именно на стороне сервера, можете выполнить команды: mysqladmin -u root ping или mysqladmin -u root processlist, чтобы получить от него ответ.
• Если при подключении проблема не связана с сервером, проверьте, нормально ли работает клиент. Попробуйте получить какие-либо его выходные данные для устранения неполадок.

Перевод статьи «Useful Tips to Troubleshoot Common Errors in MySQL»