SQL 交换一列中数据的值（如男女性别）

在刷LeetCode的时候遇到的SQL题目

627.Swap Salary

Given a table salary, such as the one below, that has m=male and f=female values. Swap all f and m values (i.e., change all f values to m and vice versa) with a single update query and no intermediate temp table.

For example:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | m   | 2500   |
| 2  | B    | f   | 1500   |
| 3  | C    | m   | 5500   |
| 4  | D    | f   | 500    |

After running your query, the above salary table should have the following rows:

| id | name | sex | salary |
|----|------|-----|--------|
| 1  | A    | f   | 2500   |
| 2  | B    | m   | 1500   |
| 3  | C    | f   | 5500   |
| 4  | D    | m   | 500    |

大概就是要求交换表中sex列的性别

一开始想着用update salary set sex = f when sex = m再用个AND，后来一想这样执行的结果应该是错的、

然后就偷看了solution，才知道还有when case语句，所以记录一下

update salary set sex =   
    case sex  
        when 'm' then 'f'  
        else 'm'  
    end; 

大概意思就是when m，then就设置为f，else都设置为m