# md函数笔记五

注：笔记旨在记录

五、MySQL 流程控制函数

0.表

0.1 num表:

CREATE TABLE `anyot`.`Untitled`  (
`id` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
`rand_int_num` int NULL DEFAULT NULL,
`rand_decimal_num` decimal(65, 2) NULL DEFAULT NULL,
`rand_string_num` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

1. IF() -- 条件判断

1.1. 函数：

• IF(expr1,expr2,expr3) : expr1为判断条件，符合则返回expr2，否则expr3

1.2. sql示例：

SELECT rand_int_num,IF(rand_int_num < 15,"√","×") from num

2. IFNULL() -- 判空判断

2.1. 函数：

• IFNULL(expr1,expr2) :expr1为判断的值或字段，为空则返回expr2

2.2. sql示例：

SELECT rand_int_num,IFNULL(rand_int_num,"√") from num

3. CASE -- 求总数

3.1. 函数：

注: case结合when,else,end使用;其中else可以省略，但end一定要加。

• 写法一：

    CASE case_value 
        WHEN when_value THEN statement_list
        WHEN ...
        ELSE statement_list
    END;
  

• 写法二：

    CASE 
        WHEN expr1 THEN statement_list
        WHEN ...
        ELSE statement_list 
    END;
  

3.2. sql示例：

• 写法一：

    SELECT rand_int_num, 
    CASE rand_int_num 
        WHEN 14 THEN " == 14"
        WHEN 44 THEN " == 44"
        ELSE " != 14"
    END as is14
    from num
  

• 写法二：

    SELECT rand_int_num,
    CASE WHEN rand_int_num = 14 THEN " == 14"
        ELSE " != 14" 
    END as is14 ,rand_string_num,
    CASE WHEN rand_string_num = "a99" THEN " == a99"
        ELSE " != a99"
    END as isa99 from num
  

    SELECT CASE 
        WHEN WEEKDAY(NOW())=0 THEN '星期一' 
        WHEN WEEKDAY(NOW())=1 THEN '星期二'  
        WHEN WEEKDAY(NOW())=2 THEN '星期三' 
        WHEN WEEKDAY(NOW())=3 THEN '星期四' 
        WHEN WEEKDAY(NOW())=4 THEN '星期五'
        WHEN WEEKDAY(NOW())=5 THEN '星期六' 
        WHEN WEEKDAY(NOW())=6 THEN '星期天' 
    END AS COLUMN1,NOW(),WEEKDAY(NOW()),DAYNAME(NOW());