CASE

声明

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

说明

在第一种声明的返回结果中，value = compare-value。而第二种声明的返回结果是第一种条件为真。如果没有匹配的结果值，则返回结果为 ELSE 后的结果，如果没有 ELSE 部分，则返回值为 NULL。

示例

SELECT CASE 'B' WHEN 'A' THEN 1 WHEN 'B' THEN 2 END;
+----------------------------------------------+
| CASE 'B' WHEN 'A' THEN 1 WHEN 'B' THEN 2 END |
+----------------------------------------------+
|                                            2 |
+----------------------------------------------+
1 row in set (0.001 sec)

SELECT CASE CONCAT('A','B') WHEN CONCAT('AB','') THEN 'A' WHEN 'B' THEN 'B' END;
+--------------------------------------------------------------------------+
| CASE CONCAT('A','B') WHEN CONCAT('AB','') THEN 'A' WHEN 'B' THEN 'B' END |
+--------------------------------------------------------------------------+
| A                                                                        |
+--------------------------------------------------------------------------+
1 row in set (0.001 sec)

SELECT CASE WHEN 1>0 THEN 'TRUE' ELSE 'FALSE' END;
+--------------------------------------------+
| CASE WHEN 1>0 THEN 'TRUE' ELSE 'FALSE' END |
+--------------------------------------------+
| TRUE                                       |
+--------------------------------------------+
1 row in set (0.001 sec)