This section discusses how error messages originate within MySQL and the components they contain.
Error messages can originate on the server side or the client side:
On the server side, error messages may occur during the startup and shutdown processes, as a result of issues that occur during SQL statement execution, and so forth.
The MySQL server writes some error messages to its error log. These indicate issues of interest to database administrators or that require DBA action.
The server sends other error messages to client programs. These indicate issues pertaining only to a particular client. The MySQL client library takes errors received from the server and makes them available to the host client program.
Client-side error messages are generated from within the MySQL client library, usually involving problems communicating with the server.
Example server-side error messages written to the error log:
This message produced during the startup process provides a status or progress indicator:
181009 7:43:37 [Note] Event Scheduler: scheduler thread started with id 1
This message indicates an issue that requires DBA action:
181009 3:20:39 [ERROR] Can't open the mysql.plugin table. Please run mysql_upgrade to create it.
Example server-side error message sent to client programs, as displayed by the mysql client:
mysql> SELECT * FROM no_such_table; ERROR 1146 (42S02): Table 'test.no_such_table' doesn't exist
Example client-side error message originating from within the client library, as displayed by the mysql client:
shell> mysql -h no-such-host ERROR 2005 (HY000): Unknown MySQL server host 'no-such-host' (0)
Whether an error originates from within the client library or is received from the server, a MySQL client program may respond in varying ways. As just illustrated, the client may display the error message so the user can take corrective measures. The client may instead internally attempt to resolve or retry a failed operation, or take other action.
When an error occurs, error information includes several components: an error code, SQLSTATE value, and message string. These components have the following characteristics:
Error code: This value is numeric. It is MySQL-specific and is not portable to other database systems.
Each error number has a corresponding symbolic value. Examples:
Error codes are stable across General Availability (GA) releases of a given MySQL series. Before a series reaches GA status, new codes may still be under development and are subject to change.
SQLSTATE value: This value is a five-character string (for example,
'42S02'). SQLSTATE values are taken from ANSI SQL and ODBC and are more standardized than the numeric error codes. The first two characters of an SQLSTATE value indicate the error class:
'01'indicates a warning.
'02'indicates “not found.” This is relevant within the context of cursors and is used to control what happens when a cursor reaches the end of a data set. This condition also occurs for
SELECT ... INTOstatements that retrieve no rows.
'02'indicates an exception.
For server-side errors, not all MySQL error numbers have corresponding SQLSTATE values. In these cases,
'HY000'(general error) is used.
For client-side errors, the SQLSTATE value is always
'HY000'(general error), so it is not meaningful for distinguishing one client error from another.
Message string: This string provides a textual description of the error.