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10.3.6 Character String Literal Character Set and Collation
Every character string literal has a character set and a collation.
For the simple statement SELECT
'
, the string has
the connection default character set and collation defined by
the string
'character_set_connection
and collation_connection
system
variables.
A character string literal may have an optional character set
introducer and COLLATE
clause, to designate
it as a string that uses a particular character set and
collation:
The _
expression is formally called an
introducer. It tells the parser, “the
string that follows uses character set
charset_name
charset_name
.” An introducer
does not change the string to the introducer character set like
CONVERT()
would do. It does not
change the string value, although padding may occur. The
introducer is just a signal. See
Section 10.3.8, “Character Set Introducers”.
Examples:
Character set introducers and the COLLATE
clause are implemented according to standard SQL specifications.
MySQL determines the character set and collation of a character string literal in the following manner:
If both
_charset_name
andCOLLATE
are specified, character setcollation_name
charset_name
and collationcollation_name
are used.collation_name
must be a permitted collation forcharset_name
.If
_charset_name
is specified butCOLLATE
is not specified, character setcharset_name
and its default collation are used. To see the default collation for each character set, use theSHOW CHARACTER SET
statement or query theINFORMATION_SCHEMA
CHARACTER_SETS
table.If
_charset_name
is not specified butCOLLATE
is specified, the connection default character set given by thecollation_name
character_set_connection
system variable and collationcollation_name
are used.collation_name
must be a permitted collation for the connection default character set.Otherwise (neither
_charset_name
norCOLLATE
is specified), the connection default character set and collation given by thecollation_name
character_set_connection
andcollation_connection
system variables are used.
Examples:
A nonbinary string with
latin1
character set andlatin1_german1_ci
collation:A nonbinary string with
utf8mb4
character set and its default collation (that is,utf8mb4_general_ci
):A binary string with
binary
character set and its default collation (that is,binary
):A nonbinary string with the connection default character set and
utf8mb4_general_ci
collation (fails if the connection character set is notutf8mb4
):A string with the connection default character set and collation:
An introducer indicates the character set for the following
string, but does not change how the parser performs escape
processing within the string. Escapes are always interpreted by
the parser according to the character set given by
character_set_connection
.
The following examples show that escape processing occurs using
character_set_connection
even
in the presence of an introducer. The examples use
SET NAMES
(which changes
character_set_connection
, as
discussed in Section 10.4, “Connection Character Sets and Collations”), and display
the resulting strings using the
HEX()
function so that the exact
string contents can be seen.
Example 1:
- +------------+-----------------+
- +------------+-----------------+
- | E00A | E00A |
- +------------+-----------------+
Here, à
(hexadecimal value
E0
) is followed by \n
, the
escape sequence for newline. The escape sequence is interpreted
using the
character_set_connection
value
of latin1
to produce a literal newline
(hexadecimal value 0A
). This happens even for
the second string. That is, the _sjis
introducer does not affect the parser's escape processing.
Example 2:
- +------------+-------------------+
- +------------+-------------------+
- | E05C6E | E05C6E |
- +------------+-------------------+
Here, character_set_connection
is sjis
, a character set in which the
sequence of à
followed by
\
(hexadecimal values 05
and 5C
) is a valid multibyte character.
Hence, the first two bytes of the string are interpreted as a
single sjis
character, and the
\
is not interpreted as an escape character.
The following n
(hexadecimal value
6E
) is not interpreted as part of an escape
sequence. This is true even for the second string; the
_latin1
introducer does not affect escape
processing.
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Dokument erstellt 26/06/2006, zuletzt geändert 26/10/2018
Quelle des gedruckten Dokuments:https://www.gaudry.be/de/mysql-rf-charset-literal.html
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