MySQL :: MySQL 8.4 Reference Manual :: 14.5 Flow Control Functions

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MySQL 8.4 Reference Manual / Functions and Operators / Flow Control Functions

14.5 流程控制函数

Table 14.7 Flow Control Operators

Name	Description
`CASE`	Case operator
`IF()`	If/else construct
`IFNULL()`	Null if/else construct
`NULLIF()`	Return NULL if expr1 = expr2

CASE value WHEN compare_value THEN result [WHEN compare_value THEN result ...] [ELSE result] END

CASE WHEN condition THEN result [WHEN condition THEN result ...] [ELSE result] END

The first CASE syntax returns the result for the first value=compare_value comparison that is true. The second syntax returns the result for the first condition that is true. If no comparison or condition is true, the result after ELSE is returned, or NULL if there is no ELSE part.

Note

The syntax of the CASE operator described here differs slightly from that of the SQL CASE statement described in Section 15.6.5.1, “CASE Statement”, for use inside stored programs. The CASE statement cannot have an ELSE NULL clause, and it is terminated with END CASE instead of END.

The return type of a CASE expression result is the aggregated type of all result values:
- If all types are numeric, the aggregated type is also numeric:
  - If at least one argument is double precision, the result is double precision.
  - Otherwise, if at least one argument is DECIMAL, the result is DECIMAL.
  - Otherwise, the result is an integer type (with one exception):
    
    If all integer types are all signed or all unsigned, the result is the same sign and the precision is the highest of all specified integer types (that is, TINYINT, SMALLINT, MEDIUMINT, INT, or BIGINT).
    
    If there is a combination of signed and unsigned integer types, the result is signed and the precision may be higher. For example, if the types are signed INT and unsigned INT, the result is signed BIGINT.
    
    The exception is unsigned BIGINT combined with any signed integer type. The result is DECIMAL with sufficient precision and scale 0.
- If all types are BIT, the result is BIT. Otherwise, BIT arguments are treated similar to BIGINT.
- If all types are YEAR, the result is YEAR. Otherwise, YEAR arguments are treated similar to INT.
- If all types are character string (CHAR or VARCHAR), the result is VARCHAR with maximum length determined by the longest character length of the operands.
- If all types are character or binary string, the result is VARBINARY.
- SET and ENUM are treated similar to VARCHAR; the result is VARCHAR.
- If all types are JSON, the result is JSON.
- If all types are temporal, the result is temporal:
  - If all temporal types are DATE, TIME, or TIMESTAMP, the result is DATE, TIME, or TIMESTAMP, respectively.
  - Otherwise, for a mix of temporal types, the result is DATETIME.
- If all types are GEOMETRY, the result is GEOMETRY.
- If any type is BLOB, the result is BLOB.
- For all other type combinations, the result is VARCHAR.
- Literal NULL operands are ignored for type aggregation.
```
mysql> SELECT CASE 1 WHEN 1 THEN 'one'
    ->     WHEN 2 THEN 'two' ELSE 'more' END;
        -> 'one'
mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
        -> 'true'
mysql> SELECT CASE BINARY 'B'
    ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
        -> NULL
```
IF(expr1,expr2,expr3)

If expr1 is TRUE (expr1 <> 0 and expr1 IS NOT NULL), IF() returns expr2. Otherwise, it returns expr3.

Note

There is also an IF statement, which differs from the IF() function described here. See Section 15.6.5.2, “IF Statement”.

If only one of expr2 or expr3 is explicitly NULL, the result type of the IF() function is the type of the non-NULL expression.

The default return type of IF() (which may matter when it is stored into a temporary table) is calculated as follows:
- If expr2 or expr3 produce a string, the result is a string.
  
  If expr2 and expr3 are both strings, the result is case-sensitive if either string is case-sensitive.
- If expr2 or expr3 produce a floating-point value, the result is a floating-point value.
- If expr2 or expr3 produce an integer, the result is an integer.
```
mysql> SELECT IF(1>2,2,3);
        -> 3
mysql> SELECT IF(1<2,'yes','no');
        -> 'yes'
mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
        -> 'no'
```

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

mysql> SELECT IFNULL(1,0);
        -> 1
mysql> SELECT IFNULL(NULL,10);
        -> 10
mysql> SELECT IFNULL(1/0,10);
        -> 10
mysql> SELECT IFNULL(1/0,'yes');
        -> 'yes'

The default return type of IFNULL(expr1,expr2) is the more “general” of the two expressions, in the order STRING, REAL, or INTEGER. Consider the case of a table based on expressions or where MySQL must internally store a value returned by IFNULL() in a temporary table:

mysql> CREATE TABLE tmp SELECT IFNULL(1,'test') AS test;
mysql> DESCRIBE tmp;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| test  | varbinary(4) | NO   |     |         |       |
+-------+--------------+------+-----+---------+-------+

In this example, the type of the test column is VARBINARY(4) (a string type).

NULLIF(expr1,expr2)

Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

The return value has the same type as the first argument.
```
mysql> SELECT NULLIF(1,1);
        -> NULL
mysql> SELECT NULLIF(1,2);
        -> 1
```
Note

MySQL evaluates expr1 twice if the arguments are not equal.

For each of these functions, if the first argument contains only characters present in the character set and collation used by the second argument (and it is constant), the latter character set and collation is used to make the comparison. System variable values are handled as column values of the same character and collation. Some queries using these functions with system variables may be rejected with Illegal mix of collations as a result. In such cases, you should cast the system variable to the correct character set and collation.

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