syntax：
IF(expr,if_true_expr,if_false_expr)
syntax：
1 CASE value
2 WHEN compare_value_1 THEN result_1
3 WHEN compare_value_2 THEN result_2
4 …
5 ELSE result END
second form：
1 CASE
2 WHEN condition_1 THEN result_1
3 WHEN condition_2 THEN result_2
4 …
5 ELSE result END
syntax:
1 CASE case_expression
2 WHEN when_expression_1 THEN commands
3 WHEN when_expression_2 THEN commands
4 ...
5 ELSE commands
6 END CASE;