md函数笔记五
注：笔记旨在记录
五、MySQL 流程控制函数
\ 函 数 名 称 作 用 完 成 1 IF
条件判断 勾 2 IFNULL
判空判断 勾 3 CASE
求数量 勾
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());
Original: https://www.cnblogs.com/hmpn/p/16474020.html
Author: hmpn
Title: MySQL函数学习(五)—–流程控制函数
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