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    |

sql：
create table if not exists salary(id int, name varchar(100), sex char(1), salary int);
Truncate table salary;
insert into salary (id, name, sex, salary) values ('1', 'A', 'm', '2500');
insert into salary (id, name, sex, salary) values ('2', 'B', 'f', '1500');
insert into salary (id, name, sex, salary) values ('3', 'C', 'm', '5500');
insert into salary (id, name, sex, salary) values ('4', 'D', 'f', '500');

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.

最后一句 and no intermediate temp table 没有中间临时表进行交换

我直接全段翻译的结果是：

坑。。。

解题：
1 . update salary set sex = case when sex='m' then 'f' else 'm' end
2 . update salary set sex = if(sex = 'f','m','f');