MySQL入门命令之函数-单行函数-流程控制函数

1、if函数
if(表达式1,表达式2,表达式3) 如果表达式1成立则执行表达式2，否则执行表达式3。

mysql> select if(5>=5,'true','false');
+-------------------------+
| if(5>=5,'true','false') |
+-------------------------+
| true                    |
+-------------------------+
1 row in set (0.00 sec)
#实例
mysql> select last_name,commission_pct,if(commission_pct is null,'没奖金，呵呵','有奖金，嘻嘻') as beizhu
    -> from employees
    -> where commission_pct is null and salary>=17000 or commission_pct>=0.4;
+-----------+----------------+--------------+
| last_name | commission_pct | beizhu       |
+-----------+----------------+--------------+
| K_ing     |           NULL | 没奖金，呵呵 |
| Kochhar   |           NULL | 没奖金，呵呵 |
| De Haan   |           NULL | 没奖金，呵呵 |
| Russell   |           0.40 | 有奖金，嘻嘻 |
+-----------+----------------+--------------+
4 rows in set (0.00 sec)

2、case
case 要判断的字段或函数表达式
when 常量1 then 要执行的语句或值;（为值是不要分号）
…
else 默认情况下执行;
end

示例一
mysql> select salary as 原始工资,department_id,
    -> case department_id
    -> when 30 then salary*1.1
    -> when 40 then salary*1.2
    -> when 50 then salary*1.3
    -> end as 新工资
    -> from employees
    -> where department_id in(30,40,50) and salary>7500;
+----------+---------------+----------+
| 原始工资 | department_id | 新工资   |
+----------+---------------+----------+
| 11000.00 |            30 | 12100.00 |
|  8000.00 |            50 | 10400.00 |
|  8200.00 |            50 | 10660.00 |
|  7900.00 |            50 | 10270.00 |
+----------+---------------+----------+
4 rows in set (0.00 sec)
示例二
mysql> SELECT last_name,salary,
    -> CASE
    -> WHEN salary>20000 THEN 'A'
    -> WHEN salary>15000 THEN 'B'
    -> WHEN salary>10000 THEN 'C'
    -> END AS dj
    -> FROM employees
    -> WHERE salary>=17000;
+-----------+----------+------+
| last_name | salary   | dj   |
+-----------+----------+------+
| K_ing     | 24000.00 | A    |
| Kochhar   | 17000.00 | B    |
| De Haan   | 17000.00 | B    |
+-----------+----------+------+
3 rows in set (0.00 sec)