This document provides a reference for the various types of error messages in MySQL:
Error messages produced only by the server: Chapter 2, Server Error Message Reference
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.
Error messages that originate only from within the MySQL client library: Chapter 3, Client Error Message Reference
Client-side error messages are generated from within the MySQL client library, usually involving problems communicating with the server.
Error messages that are shared between the server and the client library: Chapter 4, Global Error Message Reference
Some “global” error messages are shared in the sense that they can be produced by the MySQL server or by MySQL client programs.