There are several sources of error information in MySQL:
Each SQL statement executed results in an error code, an SQLSTATE value, and an error message, as described in Section B.2, “Types of Error Values”. These errors are returned from the server side; see Section B.3, “Server Error Codes and Messages”.
Errors can occur on the client side, usually involving problems communicating with the server; see Section B.4, “Client Error Codes and Messages”.
SQL statement warning and error information is available via the SHOW WARNINGS and SHOW ERRORS statements. The warning_count and error_count system variables provide counts of the number of warnings and errors.
SHOW SLAVE STATUS statement output includes information about replication errors occurring on the slave side.
SHOW ENGINE INNODB STATUS statement output includes information about the most recent foreign key error if a CREATE TABLE statement for an InnoDB table fails.
The perror program provides information from the command line about error numbers. See Section 4.8.1, “perror — Explain Error Codes”.
Descriptions of server and client errors are provided later in this Appendix. For information about errors related to InnoDB, see Section 13.6.12, “InnoDB Error Handling”.