CASE 子句

  • 项目

使用规则根据指定的条件返回特定结果，这类似于其他编程语言中的 if 和 else 语句。

语法

CASE [ expression ] { WHEN boolean_expression THEN then_expression } [ ... ]
    [ ELSE else_expression ]
END

参数

  • boolean_expression

    指定计算结果类型为 boolean 的任何表达式。 使用逻辑运算符（AND、OR）可以将两个或多个表达式组合在一起。

  • then_expression

    指明 then 表达式基于 boolean_expression 条件；then_expression 和 else_expression 应为同一类型或强制为通用类型。

  • else_expression

    指明默认表达式；then_expression 和 else_expression 应为同一类型或强制为通用类型。

示例

CREATE TABLE person (id INT, name STRING, age INT);
INSERT INTO person VALUES
    (100, 'John', 30),
    (200, 'Mary', NULL),
    (300, 'Mike', 80),
    (400, 'Dan', 50);

SELECT id, CASE WHEN id > 200 THEN 'bigger' ELSE 'small' END FROM person;
+------+--------------------------------------------------+
|  id  | CASE WHEN (id > 200) THEN bigger ELSE small END  |
+------+--------------------------------------------------+
| 100  | small                                            |
| 200  | small                                            |
| 300  | bigger                                           |
| 400  | bigger                                           |
+------+--------------------------------------------------+

SELECT id, CASE id WHEN 100 then 'bigger' WHEN  id > 300 THEN '300' ELSE 'small' END FROM person;
+------+-----------------------------------------------------------------------------------------------+
|  id  | CASE WHEN (id = 100) THEN bigger WHEN (id = CAST((id > 300) AS INT)) THEN 300 ELSE small END  |
+------+-----------------------------------------------------------------------------------------------+
| 100  | bigger                                                                                        |
| 200  | small                                                                                         |
| 300  | small                                                                                         |
| 400  | small                                                                                         |
+------+-----------------------------------------------------------------------------------------------+

SELECT * FROM person
    WHERE
        CASE 1 = 1
            WHEN 100 THEN 'big'
            WHEN 200 THEN 'bigger'
            WHEN 300 THEN 'biggest'
            ELSE 'small'
        END = 'small';
+------+-------+-------+
|  id  | name  |  age  |
+------+-------+-------+
| 100  | John  | 30    |
| 200  | Mary  | NULL  |
| 300  | Mike  | 80    |
| 400  | Dan   | 50    |
+------+-------+-------+