The repertoire of a character set is the collection of characters in the set.
String expressions have a repertoire attribute, which can have two values:
ASCII
: The expression can contain only ASCII characters; that is, characters in the Unicode rangeU+0000
toU+007F
.UNICODE
: The expression can contain characters in the Unicode rangeU+0000
toU+10FFFF
. This includes characters in the Basic Multilingual Plane (BMP) range (U+0000
toU+FFFF
) and supplementary characters outside the BMP range (U+10000
toU+10FFFF
).
The ASCII
range is a subset of
UNICODE
range, so a string with
ASCII
repertoire can be converted safely
without loss of information to the character set of any string
with UNICODE
repertoire. It can also be
converted safely to any character set that is a superset of the
ascii
character set. (All MySQL character
sets are supersets of ascii
with the
exception of swe7
, which reuses some
punctuation characters for Swedish accented characters.)
The use of repertoire enables character set conversion in expressions for many cases where MySQL would otherwise return an “illegal mix of collations” error when the rules for collation coercibility are insufficient to resolve ambiguities. (For information about coercibility, see Section 1.8.4, “Collation Coercibility in Expressions”.)
The following discussion provides examples of expressions and their repertoires, and describes how the use of repertoire changes string expression evaluation:
The repertoire for a string constant depends on string content and may differ from the repertoire of the string character set. Consider these statements:
SET NAMES utf8mb4; SELECT 'abc'; SELECT _utf8mb4'def';
Although the character set is
utf8mb4
in each of the preceding cases, the strings do not actually contain any characters outside the ASCII range, so their repertoire isASCII
rather thanUNICODE
.A column having the
ascii
character set hasASCII
repertoire because of its character set. In the following table,c1
hasASCII
repertoire:CREATE TABLE t1 (c1 CHAR(1) CHARACTER SET ascii);
The following example illustrates how repertoire enables a result to be determined in a case where an error occurs without repertoire:
CREATE TABLE t1 ( c1 CHAR(1) CHARACTER SET latin1, c2 CHAR(1) CHARACTER SET ascii ); INSERT INTO t1 VALUES ('a','b'); SELECT CONCAT(c1,c2) FROM t1;
Without repertoire, this error occurs:
ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,IMPLICIT) and (ascii_general_ci,IMPLICIT) for operation 'concat'
Using repertoire, subset to superset (
ascii
tolatin1
) conversion can occur and a result is returned:+---------------+ | CONCAT(c1,c2) | +---------------+ | ab | +---------------+
Functions with one string argument inherit the repertoire of their argument. The result of
UPPER(_utf8mb4'abc')
hasASCII
repertoire because its argument hasASCII
repertoire. (Despite the_utf8mb4
introducer, the string'abc'
contains no characters outside the ASCII range.)For functions that return a string but do not have string arguments and use
character_set_connection
as the result character set, the result repertoire isASCII
ifcharacter_set_connection
isascii
, andUNICODE
otherwise:FORMAT(numeric_column, 4);
Use of repertoire changes how MySQL evaluates the following example:
SET NAMES ascii; CREATE TABLE t1 (a INT, b VARCHAR(10) CHARACTER SET latin1); INSERT INTO t1 VALUES (1,'b'); SELECT CONCAT(FORMAT(a, 4), b) FROM t1;
Without repertoire, this error occurs:
ERROR 1267 (HY000): Illegal mix of collations (ascii_general_ci,COERCIBLE) and (latin1_swedish_ci,IMPLICIT) for operation 'concat'
With repertoire, a result is returned:
+-------------------------+ | CONCAT(FORMAT(a, 4), b) | +-------------------------+ | 1.0000b | +-------------------------+
Functions with two or more string arguments use the “widest” argument repertoire for the result repertoire, where
UNICODE
is wider thanASCII
. Consider the followingCONCAT()
calls:CONCAT(_ucs2 X'0041', _ucs2 X'0042') CONCAT(_ucs2 X'0041', _ucs2 X'00C2')
For the first call, the repertoire is
ASCII
because both arguments are within the ASCII range. For the second call, the repertoire isUNICODE
because the second argument is outside the ASCII range.The repertoire for function return values is determined based on the repertoire of only those arguments that affect the result's character set and collation.
IF(column1 < column2, 'smaller', 'greater')
The result repertoire is
ASCII
because the two string arguments (the second argument and the third argument) both haveASCII
repertoire. The first argument does not matter for the result repertoire, even if the expression uses string values.