MySQL :: MySQL 8.0 Reference Manual :: 12.8.1 String Comparison Functions and Operators

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12.8.1 String Comparison Functions and Operators

Table 12.13 String Comparison Functions and Operators

Name	Description
`LIKE`	Simple pattern matching
`不喜欢`	Negation of simple pattern matching
`STRCMP()`	Compare two strings

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This affects only comparisons.

Normally, if any expression in a string comparison is case-sensitive, the comparison is performed in case-sensitive fashion.

If a string function is invoked from within themysqlclient, binary strings display using hexadecimal notation, depending on the value of the--binary-as-hex. For more information about that option, see4.5.1节,“mysql——mysql命令行Client”.

exprLIKEpat[ESCAPE 'escape_char']
Pattern matching using an SQL pattern. Returns1(TRUE) or0(FALSE). If eitherexprorpatisNULL, the result isNULL.
The pattern need not be a literal string. For example, it can be specified as a string expression or table column. In the latter case, the column must be defined as one of the MySQL string types (seeSection 11.3, “String Data Types”).
Per the SQL standard,LIKEperforms matching on a per-character basis, thus it can produce results different from the=comparison operator:
```
mysql >选择“a”像核对latin1_german2 ae_ci; +-----------------------------------------+ | 'ä' LIKE 'ae' COLLATE latin1_german2_ci | +-----------------------------------------+ | 0 | +-----------------------------------------+ mysql> SELECT 'ä' = 'ae' COLLATE latin1_german2_ci; +--------------------------------------+ | 'ä' = 'ae' COLLATE latin1_german2_ci | +--------------------------------------+ | 1 | +--------------------------------------+
```
In particular, trailing spaces are always significant. This differs from comparisons performed with the=operator, for which the significance of trailing spaces in nonbinary strings (CHAR,VARCHAR, andTEXTvalues) depends on the pad attribute of the the collation used for the comparison. For more information, seeTrailing Space Handling in Comparisons.
WithLIKEyou can use the following two wildcard characters in the pattern:
- %matches any number of characters, even zero characters.
- _matches exactly one character.
```
mysql> SELECT 'David!' LIKE 'David_'; -> 1 mysql> SELECT 'David!' LIKE '%D%v%'; -> 1
```
To test for literal instances of a wildcard character, precede it by the escape character. If you do not specify theESCAPEcharacter,\is assumed, unless theNO_BACKSLASH_ESCAPESSQL mode is enabled. In that case, no escape character is used.
- \%matches one%character.
- \_matches one_character.
```
mysql> SELECT 'David!' LIKE 'David\_'; -> 0 mysql> SELECT 'David_' LIKE 'David\_'; -> 1
```
To specify a different escape character, use theESCAPEclause:
```
mysql> SELECT 'David_' LIKE 'David|_' ESCAPE '|'; -> 1
```
The escape sequence should be one character long to specify the escape character, or empty to specify that no escape character is used. The expression must evaluate as a constant at execution time. If theNO_BACKSLASH_ESCAPESSQL mode is enabled, the sequence cannot be empty.
The following two statements illustrate that string comparisons are not case-sensitive unless one of the operands is case-sensitive (uses a case-sensitive collation or is a binary string):
```
mysql> SELECT 'abc' LIKE 'ABC'; -> 1 mysql> SELECT 'abc' LIKE _utf8mb4 'ABC' COLLATE utf8mb4_0900_as_cs; -> 0 mysql> SELECT 'abc' LIKE _utf8mb4 'ABC' COLLATE utf8mb4_bin; -> 0 mysql> SELECT 'abc' LIKE BINARY 'ABC'; -> 0
```
As an extension to standard SQL, MySQL permitsLIKEon numeric expressions.
```
mysql> SELECT 10 LIKE '1%'; -> 1
```
Note

MySQL uses C escape syntax in strings (for example,\nto represent the newline character). If you want aLIKEstring to contain a literal\, you must double it. (Unless theNO_BACKSLASH_ESCAPESSQL mode is enabled, in which case no escape character is used.) For example, to search for\n, specify it as\\n. To search for\, specify it as\\\\; this is because the backslashes are stripped once by the parser and again when the pattern match is made, leaving a single backslash to be matched against.

Exception: At the end of the pattern string, backslash can be specified as\\. At the end of the string, backslash stands for itself because there is nothing following to escape. Suppose that a table contains the following values:
```
mysql> SELECT filename FROM t1; +--------------+ | filename | +--------------+ | C: | | C:\ | | C:\Programs | | C:\Programs\ | +--------------+
```
To test for values that end with backslash, you can match the values using either of the following patterns:
```
mysql> SELECT filename, filename LIKE '%\\' FROM t1; +--------------+---------------------+ | filename | filename LIKE '%\\' | +--------------+---------------------+ | C: | 0 | | C:\ | 1 | | C:\Programs | 0 | | C:\Programs\ | 1 | +--------------+---------------------+ mysql> SELECT filename, filename LIKE '%\\\\' FROM t1; +--------------+-----------------------+ | filename | filename LIKE '%\\\\' | +--------------+-----------------------+ | C: | 0 | | C:\ | 1 | | C:\Programs | 0 | | C:\Programs\ | 1 | +--------------+-----------------------+
```
expr不喜欢pat[ESCAPE 'escape_char']
This is the same asNOT (exprLIKEpat[ESCAPE 'escape_char']).
Note

Aggregate queries involving不喜欢comparisons with columns containingNULLmay yield unexpected results. For example, consider the following table and data:
```
CREATE TABLE foo (bar VARCHAR(10)); INSERT INTO foo VALUES (NULL), (NULL);
```
The querySELECT COUNT(*) FROM foo WHERE bar LIKE '%baz%';returns0. You might assume thatSELECT COUNT(*) FROM foo WHERE bar NOT LIKE '%baz%';would return2. However, this is not the case: The second query returns0. This is becauseNULL NOT LIKEexpralways returnsNULL, regardless of the value ofexpr. The same is true for aggregate queries involvingNULLand comparisons usingNOT RLIKEorNOT REGEXP. In such cases, you must test explicitly forNOT NULLusingOR(and notAND), as shown here:
```
SELECT COUNT(*) FROM foo WHERE bar NOT LIKE '%baz%' OR bar IS NULL;
```

STRCMP(expr1,expr2)

STRCMP()returns0if the strings are the same,-1if the first argument is smaller than the second according to the current sort order, and1otherwise.

mysql> SELECT STRCMP('text', 'text2'); -> -1 mysql> SELECT STRCMP('text2', 'text'); -> 1 mysql> SELECT STRCMP('text', 'text'); -> 0

STRCMP()performs the comparison using the collation of the arguments.

mysql> SET @s1 = _utf8mb4 'x' COLLATE utf8mb4_0900_ai_ci; mysql> SET @s2 = _utf8mb4 'X' COLLATE utf8mb4_0900_ai_ci; mysql> SET @s3 = _utf8mb4 'x' COLLATE utf8mb4_0900_as_cs; mysql> SET @s4 = _utf8mb4 'X' COLLATE utf8mb4_0900_as_cs; mysql> SELECT STRCMP(@s1, @s2), STRCMP(@s3, @s4); +------------------+------------------+ | STRCMP(@s1, @s2) | STRCMP(@s3, @s4) | +------------------+------------------+ | 0 | -1 | +------------------+------------------+

If the collations are incompatible, one of the arguments must be converted to be compatible with the other. SeeSection 10.8.4, “Collation Coercibility in Expressions”.

mysql> SET @s1 = _utf8mb4 'x' COLLATE utf8mb4_0900_ai_ci; mysql> SET @s2 = _utf8mb4 'X' COLLATE utf8mb4_0900_ai_ci; mysql> SET @s3 = _utf8mb4 'x' COLLATE utf8mb4_0900_as_cs; mysql> SET @s4 = _utf8mb4 'X' COLLATE utf8mb4_0900_as_cs; --> mysql> SELECT STRCMP(@s1, @s3); ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_0900_ai_ci,IMPLICIT) and (utf8mb4_0900_as_cs,IMPLICIT) for operation 'strcmp' mysql> SELECT STRCMP(@s1, @s3 COLLATE utf8mb4_0900_ai_ci); +---------------------------------------------+ | STRCMP(@s1, @s3 COLLATE utf8mb4_0900_ai_ci) | +---------------------------------------------+ | 0 | +---------------------------------------------+

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