CASE 子句
描述
CASE 子句根据指定条件返回特定结果，类似于其他编程语言中的 if/else 语句。
语法
CASE [ expression ] { WHEN boolean_expression THEN then_expression } [ ... ]
[ ELSE else_expression ]
END
参数
-
boolean_expression
指定任何评估结果为
boolean类型的表达式。两个或多个表达式可以使用逻辑运算符（AND,OR）组合在一起。 -
then_expression
指定基于
boolean_expression条件的 then 表达式；then_expression和else_expression应全部为相同类型或可强制转换为通用类型。 -
else_expression
指定默认表达式；
then_expression和else_expression应全部为相同类型或可强制转换为通用类型。
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 |
+------+-------+-------+