流程控制函数
- 流程处理函数可以根据不同的条件，执行不同的处理流程，可以在SQL语句中实现不同的条件选择。MySQL中的流程处理函数主要包括IF()、IFNULL()和CASE()函数。
| 函数 |
用法 |
| IF(value,value1,value2) |
如果value的值为TRUE，返回value1，否则返回value2 |
| IFNULL(value1, value2) |
如果value1不为NULL，返回value1，否则返回value2 |
| CASE WHEN 条件1 THEN 结果1 WHEN 条件2 THEN 结果2 .... [ELSE resultn] END |
相当于Java的if...else if...else... |
| CASE expr WHEN 常量值1 THEN 值1 WHEN 常量值1 THEN 值1 .... [ELSE 值n] END |
相当于Java的switch...case... |
SELECT IF(1 > 0,'正确','错误')
->正确
SELECT IFNULL(null,'Hello Word')
->Hello Word
SELECT CASE
WHEN 1 > 0
THEN '1 > 0'
WHEN 2 > 0
THEN '2 > 0'
ELSE '3 > 0'
END
->1 > 0
SELECT CASE 1
WHEN 1 THEN '我是1'
WHEN 2 THEN '我是2'
ELSE '你是谁'
SELECT employee_id,salary, CASE WHEN salary>=15000 THEN '高薪'
WHEN salary>=10000 THEN '潜力股'
WHEN salary>=8000 THEN '屌丝'
ELSE '草根' END "描述"
FROM employees;
SELECT oid,`status`, CASE `status` WHEN 1 THEN '未付款'
WHEN 2 THEN '已付款'
WHEN 3 THEN '已发货'
WHEN 4 THEN '确认收货'
ELSE '无效订单' END
FROM t_order;
mysql> SELECT CASE WHEN 1 > 0 THEN 'yes' WHEN 1 <= 0 THEN 'no' ELSE 'unknown' END;
+---------------------------------------------------------------------+
| CASE WHEN 1 > 0 THEN 'yes' WHEN 1 <= 0 THEN 'no' ELSE 'unknown' END |
+---------------------------------------------------------------------+
| yes |
+---------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT CASE WHEN 1 < 0 THEN 'yes' WHEN 1 = 0 THEN 'no' ELSE 'unknown' END;
+--------------------------------------------------------------------+
| CASE WHEN 1 < 0 THEN 'yes' WHEN 1 = 0 THEN 'no' ELSE 'unknown' END |
+--------------------------------------------------------------------+
| unknown |
+--------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT CASE 1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END;
+------------------------------------------------+
| CASE 1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END |
+------------------------------------------------+
| 1 |
+------------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT CASE -1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END;
+-------------------------------------------------+
| CASE -1 WHEN 0 THEN 0 WHEN 1 THEN 1 ELSE -1 END |
+-------------------------------------------------+
| -1 |
+-------------------------------------------------+
1 row in set (0.00 sec)
SELECT employee_id,12 * salary * (1 + IFNULL(commission_pct,0))
FROM employees;
SELECT last_name, job_id, salary,
CASE job_id WHEN 'IT_PROG' THEN 1.10*salary
WHEN 'ST_CLERK' THEN 1.15*salary
WHEN 'SA_REP' THEN 1.20*salary
ELSE salary END "REVISED_SALARY"
FROM employees;
