--流程函数。if ，ifnull ，case 语句！
mysql> create table sal (id int,sal decimal (9,2));
Query OK, 0 rows affected (0.06 sec)
mysql> insert into sal values (1,1000),(2,2000),(3,3000),(4,4000),(5,null);
Query OK, 5 rows affected (0.01 sec)
Records: 5 Duplicates: 0 Warnings: 0
--if(EXP,T,F) EXP 为真则返回T,EXP为假，则返回F
mysql> select * from sal;
+------+---------+
| id | sal |
+------+---------+
| 1 | 1000.00 |
| 2 | 2000.00 |
| 3 | 3000.00 |
| 4 | 4000.00 |
| 5 | NULL |
+------+---------+
5 rows in set (0.00 sec)
mysql> select id, if(sal > 3000,'high','low') from sal;
+------+-----------------------------+
| id | if(sal > 3000,'high','low') |
+------+-----------------------------+
| 1 | low |
| 2 | low |
| 3 | low |
| 4 | high |
| 5 | low |--这里null >3000 为假！
+------+-----------------------------+
5 rows in set (0.01 sec)
--IFNULL(VAL,N)如果val 为null 则返回N
mysql> select id, ifnull(sal,0) from sal;
+------+---------------+
| id | ifnull(sal,0) |
+------+---------------+
| 1 | 1000.00 |
| 2 | 2000.00 |
| 3 | 3000.00 |
| 4 | 4000.00 |
| 5 | 0.00 |
+------+---------------+
5 rows in set (0.01 sec)
--case 语句和oracle的一样了！
mysql> select case when sal<=3000 then 'low' else 'high' end from sal;
+------------------------------------------------+
| case when sal<=3000 then 'low' else 'high' end |
+------------------------------------------------+
| low |
| low |
| low |
| high |
| high |
+------------------------------------------------+
5 rows in set (0.00 sec)
mysql> select id, case sal when 2000 then 'low' when 3000 then 'mid' else 'high' end from sal;
+------+--------------------------------------------------------------------+
| id | case sal when 2000 then 'low' when 3000 then 'mid' else 'high' end |
+------+--------------------------------------------------------------------+
| 1 | high |
| 2 | low |
| 3 | mid |
| 4 | high |
| 5 | high |
+------+--------------------------------------------------------------------+
5 rows in set (0.01 sec)
这里只是简单介绍，跟多的需要在实践中学习！抛砖引玉了，呵呵