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控制流程函数
/docs-cn/dev/functions-and-operators/control-flow-functions/
/docs-cn/dev/reference/sql/functions-and-operators/control-flow-functions/
TiDB 支持 MySQL 8.0 中的控制流程函数，包括 CASE、IF()、IFNULL() 和 NULLIF()。这些函数可以用于构建 if/else 语句和处理 NULL 值。

控制流程函数

TiDB 支持使用 MySQL 8.0 中提供的所有控制流程函数。

函数名 功能描述
CASE Case 操作符
IF() 构建 if/else
IFNULL() 构建 Null if/else
NULLIF() 如果 expr1 = expr2，返回 NULL

CASE

CASE 操作符可以根据指定的条件进行条件逻辑判断并自定义查询结果。

语法：

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE default_result
END

示例：

WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
SELECT n, CASE WHEN n MOD 2 THEN "odd" ELSE "even" END FROM d;
+----+----------------------------------------------+
| n  | CASE WHEN n MOD 2 THEN "odd" ELSE "even" END |
+----+----------------------------------------------+
|  1 | odd                                          |
|  2 | even                                         |
|  3 | odd                                          |
|  4 | even                                         |
|  5 | odd                                          |
|  6 | even                                         |
|  7 | odd                                          |
|  8 | even                                         |
|  9 | odd                                          |
| 10 | even                                         |
+----+----------------------------------------------+
10 rows in set (0.00 sec)

IF()

IF() 函数可以根据值或表达式是否为真执行不同的操作。

语法：

IF(condition, value_if_true, value_if_false)

示例：

WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
SELECT n, IF(n MOD 2, "odd", "even") FROM d;
+----+----------------------------+
| n  | IF(n MOD 2, "odd", "even") |
+----+----------------------------+
|  1 | odd                        |
|  2 | even                       |
|  3 | odd                        |
|  4 | even                       |
|  5 | odd                        |
|  6 | even                       |
|  7 | odd                        |
|  8 | even                       |
|  9 | odd                        |
| 10 | even                       |
+----+----------------------------+
10 rows in set (0.00 sec)

IFNULL()

IFNULL(expr1,expr2) 函数用于处理查询中的 NULL 值。如果 expr1 不为 NULL，该函数返回 expr1；否则返回 expr2。

示例：

WITH data AS (SELECT NULL AS x UNION ALL SELECT 1 )
SELECT x, IFNULL(x,'x has no value') FROM data;
+------+----------------------------+
| x    | IFNULL(x,'x has no value') |
+------+----------------------------+
| NULL | x has no value             |
|    1 | 1                          |
+------+----------------------------+
2 rows in set (0.0006 sec)

NULLIF()

NULLIF(expr1,expr2) 函数用于在两个参数相同或第一个参数为 NULL 时返回 NULL。否则，返回第一个参数。

示例：

WITH RECURSIVE d AS (SELECT 1 AS n UNION ALL SELECT n+1 FROM d WHERE n<10)
SELECT n, NULLIF(n+n, n+2) FROM d;
+----+------------------+
| n  | NULLIF(n+n, n+2) |
+----+------------------+
|  1 |                2 |
|  2 |             NULL |
|  3 |                6 |
|  4 |                8 |
|  5 |               10 |
|  6 |               12 |
|  7 |               14 |
|  8 |               16 |
|  9 |               18 |
| 10 |               20 |
+----+------------------+
10 rows in set (0.00 sec)

在该示例中，当 n 等于 2 时，n+n 和 n+2 都等于 4，两个参数值相同，因此函数返回 NULL。