MySQL----case的用法

MySQLCASE表达式详解😊

CASE表达式是MySQL中非常强大的条件判断工具，它允许我们在SQL查询中进行灵活的条件分支处理。下面让我们一起来探索CASE的各种用法吧！✨

基本语法结构📝

MySQL支持两种形式的CASE表达式：

1.简单CASE表达式
```sql
SELECT
employee_name,
CASEdepartment_id
WHEN1THEN'技术部'
WHEN2THEN'市场部'
WHEN3THEN'财务部'
ELSE'其他部门'
ENDASdepartment_name
FROMemployees;
```

2.搜索CASE表达式
```sql
SELECT
product_name,
CASE
WHENprice>1000THEN'高端产品'
WHENprice>500THEN'中端产品'
WHENprice>0THEN'低端产品'
ELSE'价格异常'
ENDASproduct_level
FROMproducts;
```

实际应用场景🎯

数据分类统计
```sql
SELECT
COUNT()AStotal_orders,
SUM(CASEWHENstatus='completed'THEN1ELSE0END)AScompleted_orders,
SUM(CASEWHENstatus='pending'THEN1ELSE0END)ASpending_orders,
SUM(CASEWHENstatus='cancelled'THEN1ELSE0END)AScancelled_orders
FROMorders;
```

动态列值转换
```sql
SELECT
student_id,
name,
CASE
WHENscore>=90THEN'A'
WHENscore>=80THEN'B'
WHENscore>=70THEN'C'
WHENscore>=60THEN'D'
ELSE'F'
ENDASgrade
FROMstudents;
```

注意事项⚠️

1.CASE表达式会按顺序评估条件，第一个满足的条件会被执行，后续条件将被忽略
2.如果没有匹配的条件且没有ELSE子句，结果将为NULL
3.CASE表达式可以用在SELECT、WHERE、ORDERBY等子句中

高级用法🚀

在ORDERBY中使用
```sql
SELECTFROMproducts
ORDERBY
CASE
WHENstock<10THEN0
WHENstock<50THEN1
ELSE2
END;
```

在UPDATE中使用
```sql
UPDATEemployees
SETsalary=CASE
WHENperformance='excellent'THENsalary1.2
WHENperformance='good'THENsalary1.1
ELSEsalary1.05
END;
```

掌握CASE表达式能让你的SQL查询更加灵活强大！💪快去试试这些技巧吧！😄