ASCII: The expression can contain only ASCII characters; that is, characters in the Unicode range U+0000 to U+007F.

UNICODE: The expression can contain characters in the Unicode range U+0000 to U+10FFFF. This includes characters in the Basic Multilingual Plane (BMP) range (U+0000 to U+FFFF) and supplementary characters outside the BMP range (U+10000 to U+10FFFF).

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 is ASCII rather than UNICODE.

A column having the ascii character set has ASCII repertoire because of its character set. In the following table, c1 has ASCII 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 to latin1) 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') has ASCII repertoire because its argument has ASCII 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 is ASCII if character_set_connection is ascii, and UNICODE 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 than ASCII. Consider the following CONCAT() 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 is UNICODE 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 have ASCII repertoire. The first argument does not matter for the result repertoire, even if the expression uses string values.