This section describes how the server uses character sets for constructing error messages and returning them to clients. For information about the language of error messages (rather than the character set), see Section 10.2, "Setting the Error Message Language".
In MySQL 5.7, the server constructs error messages using UTF-8 and returns them to clients in the character set
specified by the
character_set_results system variable.
The server constructs error messages as follows:
The message template uses UTF-8.
Parameters in the message template are replaced with values that apply to a specific error occurrence:
Identifiers such as table or column names use UTF-8 internally so they are copied as is.
Character (nonbinary) string values are converted from their character set to UTF-8.
Binary string values are copied as is for bytes in the range
0x7E, and using
\x hex encoding for bytes outside that range. For example, if a
duplicate-key error occurs for an attempt to insert
VARBINARY unique column, the resulting error message uses
UTF-8 with some bytes hex encoded:
Duplicate entry 'A\xC3\x9F' for key 1
To return a message to the client after it has been constructed, the server converts it from UTF-8 to the
character set specified by the
character_set_results system variable. If
character_set_results has a value of
binary, no conversion occurs. No conversion occurs if the variable value is
utf8, either, because that matches the original error message character set.
For characters that cannot be represented in
character_set_results, some encoding may occur during the conversion. The
encoding uses Unicode code point values:
Characters in the Basic Multilingual Plane (BMP) range (
0xFFFF) are written using
Characters outside the BMP range (
0x10FFFF) are written using
Clients can set
character_set_results to control the character set in which they receive error
messages. The variable can be set directly, or indirectly by means such as
SET NAMES. For more information about
character_set_results, see Section
10.1.4, "Connection Character Sets and Collations".
The encoding that occurs during the conversion to
character_set_results before returning error messages to clients can result in
different message content compared to earlier versions (before MySQL 5.5). For example, if an error occurs for
an attempt to drop a table named
ペ (KATAKANA LETTER PE) and
character_set_results is a character set such as
latin1 that does not contain that character, the resulting message sent to the
client has an encoded table name:
ERROR 1051 (42S02): Unknown table '\30DA'
Before MySQL 5.5, the name is not encoded:
ERROR 1051 (42S02): Unknown table 'ペ'