[leetcode/db] 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    |

Using the keyword CASE could solve the problem easily.

UPDATE salary
SET
sex = CASE sex
WHEN 'f' THEN 'm'
ELSE 'f'
END;

CASE evaluates a list of conditions and returns one of multiple possible result expressions.
It has two formats:

• The simple CASE expression, which compares an expression to a set of simple expressions to determine the result.

  CASE input_expression
  WHEN when_expression THEN result_expression [ ...n ]
  [ ELSE else_result_expression ]
  END
  

• The searched CASE expression, which evaluates a set of Boolean expressions to determine the result.

  CASE
  WHEN Boolean_expression THEN result_expression [ ...n ]
  [ ELSE else_result_expression ]
  END
  

CASE is very similar to ‘case’ expressions in other programming languagues.
For this problem, we use the simple CASE expression, but in fact both could be applied.