字符字符串字面量、十六进制字面量或位值字面量可以有可选的字符集引入器和 COLLATE 子句，以将其指定为使用特定字符集和排序规则的字符串：A character string literal, hexadecimal literal, or bit-value 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:

[_charset_name] literal [COLLATE collation_name]

_charset_name 表达式正式称为 引导符。它告诉解析器，“后续的字符串使用字符集 charset_name。” 引导符并不会像 CONVERT() 那样将字符串转换为引导符指定的字符集。它不会改变字符串的值，尽管可能会发生填充。引导符只是一个信号。The _charset_name expression is formally called an introducer. It tells the parser, “the string that follows uses character set 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.

对于字符字符串字面量，介绍符和字符串之间可以有空格，但这是可选的。For character string literals, space between the introducer and the string is permitted but optional.

对于字符集字面量，引导符指示以下字符串的字符集，但不会改变解析器在字符串内执行转义处理的方式。解析器始终根据character_set_connection指定的字符集解释转义。有关更多讨论和示例，请参见第 12.3.6 节，“字符串字面量的字符集和排序规则”。For character set literals, 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. For additional discussion and examples, see Section 12.3.6, “Character String Literal Character Set and Collation”.

示例：Examples:

SELECT 'abc';
SELECT _latin1'abc';
SELECT _binary'abc';
SELECT _utf8mb4'abc' COLLATE utf8mb4_danish_ci;

SELECT _latin1 X'4D7953514C';
SELECT _utf8mb4 0x4D7953514C COLLATE utf8mb4_danish_ci;

SELECT _latin1 b'1000001';
SELECT _utf8mb4 0b1000001 COLLATE utf8mb4_danish_ci;

字符集引入符和 COLLATE 子句是根据标准 SQL 规范实现的。Character set introducers and the COLLATE clause are implemented according to standard SQL specifications.

字符字符串字面量可以通过使用 _binary 引入标记为二进制字符串。十六进制字面量和位值字面量默认就是二进制字符串，因此允许使用 _binary，但通常不必要。在某些情况下，_binary 可能有助于将十六进制或位字面量保留为二进制字符串，否则这些字面量会被视为数字。例如，在 MySQL 8.4 及更高版本中，位操作允许数字或二进制字符串参数，但默认将十六进制和位字面量视为数字。要明确为此类字面量指定二进制字符串上下文，请在至少一个参数上使用 _binary 引入标记：Character string literals can be designated as binary strings by using the _binary introducer. Hexadecimal literals and bit-value literals are binary strings by default, so _binary is permitted, but normally unnecessary. _binary may be useful to preserve a hexadecimal or bit literal as a binary string in contexts for which the literal is otherwise treated as a number. For example, bit operations permit numeric or binary string arguments in MySQL 8.4 and higher, but treat hexadecimal and bit literals as numbers by default. To explicitly specify binary string context for such literals, use a _binary introducer for at least one of the arguments:

mysql> SET @v1 = X'000D' | X'0BC0';
mysql> SET @v2 = _binary X'000D' | X'0BC0';
mysql> SELECT HEX(@v1), HEX(@v2);
+----------+----------+
| HEX(@v1) | HEX(@v2) |
+----------+----------+
| BCD      | 0BCD     |
+----------+----------+

显示的结果对于这两种位操作看起来相似，但没有 _binary 的结果是 BIGINT 值，而使用 _binary 的结果是二进制字符串。由于结果类型的不同，显示的值也不同：数字结果不会显示高位的 0。The displayed result appears similar for both bit operations, but the result without _binary is a BIGINT value, whereas the result with _binary is a binary string. Due to the difference in result types, the displayed values differ: High-order 0 digits are not displayed for the numeric result.

MySQL 按如下方式确定字符字符串字面值、十六进制字面值或位值字面值的字符集和排序规则：MySQL determines the character set and collation of a character string literal, hexadecimal literal, or bit-value literal in the following manner:

示例：Examples: