建表及插入数据语句： mysql> create table salary(userid int,salary decimal(9,2)); Query OK, 0 rows affected (0.11 sec) mysql> insert into salary values(1,1000),(2,2000),(3,3000),(4,4000),(5,5000),(1, null); Query OK, 6 rows affected (0.13 sec) Records: 6 Duplicates: 0 Warnings: 0 1.if(value,t,f)函数:这里认为高于2000就是'high',其他都是low mysql> select if(salary>2000,'high','low') from salary; +------------------------------+ | if(salary>2000,'high','low') | +------------------------------+ | low | | low | | high | | high | | high | | low | +------------------------------+ 6 rows in set (0.00 sec) 2.ifnull(value1,value2)函数:这个函数可以用来将NULL值换成0 mysql> select ifnull(salary,0) from salary; +------------------+ | ifnull(salary,0) | +------------------+ | 1000.00 | | 2000.00 | | 3000.00 | | 4000.00 | | 5000.00 | | 0.00 | +------------------+ 6 rows in set (0.00 sec) 3.case when [value1] then [result]...else [default] end函数: mysql> select case when salary<=2000 then 'low' else 'high' end from salary; +---------------------------------------------------+ | case when salary<=2000 then 'low' else 'high' end | +---------------------------------------------------+ | low | | low | | high | | high | | high | | high | +---------------------------------------------------+ 6 rows in set (0.00 sec) 4.case [expr] when [value1] then [result] ... else [default] end函数: mysql> select case salary when 1000 then 'low' when 2000 then 'mid' else 'high'e nd from salary; +----------------------------------------------------------------------+ | case salary when 1000 then 'low' when 2000 then 'mid' else 'high'end | +----------------------------------------------------------------------+ | low | | mid | | high | | high | | high | | high | +----------------------------------------------------------------------+ 6 rows in set (0.02 sec) 5.关于mysql存储引擎的一些东西: 存储引擎是mysql不同于其他数据库的一个重要特性，用户可以根据实际需要利用这个特性定制自己的存储引擎. mysql的引擎有: mysql> show engines \G; *************************** 1. row *************************** Engine: MyISAM Support: YES Comment: Default engine as of MySQL 3.23 with great performance *************************** 2. row *************************** Engine: MEMORY Support: YES Comment: Hash based, stored in memory, useful for temporary tables *************************** 3. row *************************** Engine: InnoDB Support: DEFAULT Comment: Supports transactions, row-level locking, and foreign keys *************************** 4. row *************************** Engine: BerkeleyDB Support: NO Comment: Supports transactions and page-level locking *************************** 5. row *************************** Engine: BLACKHOLE Support: NO Comment: /dev/null storage engine (anything you write to it disappears) *************************** 6. row *************************** Engine: EXAMPLE Support: NO Comment: Example storage engine *************************** 7. row *************************** Engine: ARCHIVE Support: YES Comment: Archive storage engine *************************** 8. row *************************** Engine: CSV Support: NO Comment: CSV storage engine *************************** 9. row *************************** Engine: ndbcluster Support: NO Comment: Clustered, fault-tolerant, memory-based tables *************************** 10. row *************************** Engine: FEDERATED Support: NO Comment: Federated MySQL storage engine *************************** 11. row *************************** Engine: MRG_MYISAM Support: YES Comment: Collection of identical MyISAM tables *************************** 12. row *************