MySQL中的case表达式

case 表达式 & case 语句

假如我们有一张表名为 employees ，如下所示：

+-------------+------------+-----------+---------------+---------+------------+--------+
| employee_id | first_name | last_name | department_id | salary  | hire_date  | gender |
+-------------+------------+-----------+---------------+---------+------------+--------+
|           1 | John       | Doe       |             1 | 4500.00 | 2020-01-15 | M      |
|           2 | Jane       | Smith     |             2 | 5500.00 | 2019-03-22 | F      |
|           3 | Robert     | Brown     |             1 | 3500.00 | 2018-07-11 | M      |
|           4 | Emily      | Davis     |             3 | 2500.00 | 2021-05-17 | F      |
|           5 | Michael    | Wilson    |             2 | 7000.00 | 2017-11-05 | M      |
|           6 | Linda      | Taylor    |             3 | 4200.00 | 2020-10-25 | F      |
|           7 | David      | Anderson  |             1 | 4800.00 | 2019-08-19 | M      |
|           8 | Sarah      | Thomas    |             2 | 5100.00 | 2018-12-13 | F      |
|           9 | James      | Jackson   |             3 | 3100.00 | 2016-09-03 | M      |
|          10 | Patricia   | White     |             1 | 6000.00 | 2015-02-22 | F      |
+-------------+------------+-----------+---------------+---------+------------+--------+

case 表达式

CASE 表达式是用于在 SELECT 或其他查询语句中根据条件产生不同结果的方法。case 表达式有简单 case 表达式和搜索 case 表达式。（了解即可，实际上输出的结果是相同的）

基于员工的薪水（salary）字段返回不同的级别：

-- 搜索 case 表达式
SELECT first_name, last_name,CASEWHEN salary < 3000 THEN 'Low'WHEN salary >= 3000 AND salary < 5000 THEN 'Medium'WHEN salary >= 5000 THEN 'High'ELSE 'Unknown'END AS salary_level
FROM employees;+------------+-----------+--------------+
| first_name | last_name | salary_level |
+------------+-----------+--------------+
| John       | Doe       | Medium       |
| Jane       | Smith     | High         |
| Robert     | Brown     | Medium       |
| Emily      | Davis     | Low          |
| Michael    | Wilson    | High         |
| Linda      | Taylor    | Medium       |
| David      | Anderson  | Medium       |
| Sarah      | Thomas    | High         |
| James      | Jackson   | Medium       |
| Patricia   | White     | High         |
+------------+-----------+--------------+

-- 简单 case 表达式
SELECT first_name,last_name,CASE department_idWHEN 1 THEN 'HR'WHEN 2 THEN 'Engineering'WHEN 3 THEN 'Sales'ELSE 'Unknown'END AS department_name
FROM employees;+------------+-----------+-----------------+
| first_name | last_name | department_name |
+------------+-----------+-----------------+
| John       | Doe       | HR              |
| Jane       | Smith     | Engineering     |
| Robert     | Brown     | HR              |
| Emily      | Davis     | Sales           |
| Michael    | Wilson    | Engineering     |
| Linda      | Taylor    | Sales           |
| David      | Anderson  | HR              |
| Sarah      | Thomas    | Engineering     |
| James      | Jackson   | Sales           |
| Patricia   | White     | HR              |
+------------+-----------+-----------------+
10 rows in set (0.00 sec)

注意点：

else 子句可以不用写，但是不建议。养成良好的习惯。
end 是必不可少的。

case 语句

CASE 语句是一种更通用的形式，可以在 SQL 中的任何位置使用，例如 SELECT、UPDATE、DELETE 语句中。

与聚合函数结合使用。

统计每个部门的男性和女性员工数量：

SELECT department_id,SUM(CASE WHEN gender = 'M' THEN 1 ELSE 0 END) AS male_count,SUM(CASE WHEN gender = 'F' THEN 1 ELSE 0 END) AS female_count
FROM employees
GROUP BY department_id;+---------------+------------+--------------+
| department_id | male_count | female_count |
+---------------+------------+--------------+
|             1 |          3 |            1 |
|             2 |          1 |            2 |
|             3 |          1 |            2 |
+---------------+------------+--------------+
3 rows in set (0.01 sec)

case 语句不仅能写在聚合函数里面，还能写在聚合函数外面。

统计大部门和小部门：

SELECT department_id,CASE WHEN COUNT(*) > 3 THEN 'big'ELSE 'small'END AS scale
FROM employees
GROUP BY department_id;+---------------+-------+
| department_id | scale |
+---------------+-------+
|             1 | big   |
|             2 | small |
|             3 | small |
+---------------+-------+
3 rows in set (0.00 sec)

在 UPDATE 语句里进行条件分支

假如让工资大于等于 5000 的降薪 20%，小于 5000 的涨薪 20%。你会怎么做？

首先最先想到的应该是两个UPDATE进行更新。但是有没有想过执行完第一个条件后，会对第二个进行干扰，从而出现错误的结果。

下面来看一个简单的方式来完成这个要求：

UPDATE employees
SET salary = CASEWHEN salary >= 5000 THEN salary * 0.8  WHEN salary < 5000 THEN salary * 1.2 END;-- 查询后的结果
+-------------+------------+-----------+---------------+---------+------------+--------+
| employee_id | first_name | last_name | department_id | salary  | hire_date  | gender |
+-------------+------------+-----------+---------------+---------+------------+--------+
|           1 | John       | Doe       |             1 | 5400.00 | 2020-01-15 | M      |
|           2 | Jane       | Smith     |             2 | 4400.00 | 2019-03-22 | F      |
|           3 | Robert     | Brown     |             1 | 4200.00 | 2018-07-11 | M      |
|           4 | Emily      | Davis     |             3 | 3000.00 | 2021-05-17 | F      |
|           5 | Michael    | Wilson    |             2 | 5600.00 | 2017-11-05 | M      |
|           6 | Linda      | Taylor    |             3 | 5040.00 | 2020-10-25 | F      |
|           7 | David      | Anderson  |             1 | 5760.00 | 2019-08-19 | M      |
|           8 | Sarah      | Thomas    |             2 | 4080.00 | 2018-12-13 | F      |
|           9 | James      | Jackson   |             3 | 3720.00 | 2016-09-03 | M      |
|          10 | Patricia   | White     |             1 | 4800.00 | 2015-02-22 | F      |
+-------------+------------+-----------+---------------+---------+------------+--------+
10 rows in set (0.00 sec)