一、常用字符串函数
1.CONCAT(str1,str2,...)
mysql> ? concat;
mysql> SELECT CONCAT('My', 'S', 'QL');
+-------------------------+
| CONCAT('My', 'S', 'QL') |
+-------------------------+
| MySQL                   |
+-------------------------+

2.INSERT(str,pos,len,newstr)
mysql> ? insert function;
mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
+-----------------------------------+
| INSERT('Quadratic', 3, 4, 'What') |
+-----------------------------------+
| QuWhattic                         |
+-----------------------------------+

3.LOWER(str)
mysql> ? lower
mysql> SELECT LOWER('QUADRATICALLY');
+------------------------+
| LOWER('QUADRATICALLY') |
+------------------------+
| quadratically          |
+------------------------+

4.UPPER(str)
mysql> ? upper
mysql> SELECT UPPER('Hej');
+--------------+
| UPPER('Hej') |
+--------------+
| HEJ          |
+--------------+

5.LEFT(str,len)
mysql> ? left
mysql> SELECT LEFT('foobarbar', 5);
+----------------------+
| LEFT('foobarbar', 5) |
+----------------------+
| fooba                |
+----------------------+

6.RIGHT(str,len)
mysql> ? right
mysql> SELECT RIGHT('foobarbar', 4);
+-----------------------+
| RIGHT('foobarbar', 4) |
+-----------------------+
| rbar                  |
+-----------------------+

7.LPAD(str,len,padstr)
mysql> ? lpad
mysql> SELECT LPAD('hi',4,'??');
+-------------------+
| LPAD('hi',4,'??') |
+-------------------+
| ??hi              |
+-------------------+

8.RPAD(str,len,padstr)
mysql> ? rpad
mysql> SELECT RPAD('hi',5,'?');
+------------------+
| RPAD('hi',5,'?') |
+------------------+
| hi???            |
+------------------+

9.LTRIM(str)
mysql> ? ltrim
mysql> SELECT LTRIM('  barbar');
+-------------------+
| LTRIM('  barbar') |
+-------------------+
| barbar            |
+-------------------+

10.RTRIM(str)
mysql> ? rtrim
mysql> SELECT RTRIM('barbar   ');
+--------------------+
| RTRIM('barbar   ') |
+--------------------+
| barbar             |
+--------------------+

11.TRIM(str)
mysql> ? trim
mysql> SELECT TRIM('  bar   ');
+------------------+
| TRIM('  bar   ') |
+------------------+
| bar              |
+------------------+

12.REPEAT(str,count)
mysql> ? repeat function;
mysql> SELECT REPEAT('MySQL', 3);
+--------------------+
| REPEAT('MySQL', 3) |
+--------------------+
| MySQLMySQLMySQL    |
+--------------------+

13.REPLACE(str,from_str,to_str)
mysql> ? replace
mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
+-------------------------------------+
| REPLACE('www.mysql.com', 'w', 'Ww') |
+-------------------------------------+
| WwWwWw.mysql.com                    |
+-------------------------------------+

14.STRCMP(expr1,expr2)
mysql> ? strcmp
mysql> SELECT STRCMP('text', 'text2'),STRCMP('text2', 'text'),STRCMP('text', 'text');
+-----------------------+-----------------------+----------------------+
|STRCMP('text', 'text2')|STRCMP('text2', 'text')|STRCMP('text', 'text')|
+-----------------------+-----------------------+----------------------+
|                    -1 |                      1|                     0|
+-----------------------+-----------------------+----------------------+

15.SUBSTRING
mysql> ? substring
SUBSTRING(str,pos), SUBSTRING(str FROM pos), SUBSTRING(str,pos,len),
SUBSTRING(str FROM pos FOR len)
mysql> SELECT SUBSTRING('Secooler',3,4), SUBSTRING('Secooler',6);
+---------------------------+-------------------------+
| SUBSTRING('Secooler',3,4) | SUBSTRING('Secooler',6) |
+---------------------------+-------------------------+
| cool                      | ler                     |
+---------------------------+-------------------------+

二、数值函数
1.ABS(X) 取绝对值函数
mysql> ? abs
mysql> SELECT ABS(-32);
+----------+
| ABS(-32) |
+----------+
|       32 |
+----------+

2.CEILING(X), CEIL(X) 取天棚函数
mysql> ? ceil
mysql> SELECT CEILING(1.23), CEIL(-1.23);
+---------------+-------------+
| CEILING(1.23) | CEIL(-1.23) |
+---------------+-------------+
|             2 |          -1 |
+---------------+-------------+

3.FLOOR(X) 取地板函数
mysql> ? floor
mysql> SELECT FLOOR(1.23),FLOOR(-1.23);
+-------------+--------------+
| FLOOR(1.23) | FLOOR(-1.23) |
+-------------+--------------+
|           1 |           -2 |
+-------------+--------------+

4.MOD(N,M), N % M, N MOD M 取模函数
mysql> ? mod
mysql> SELECT MOD(234, 10), 253 % 7, MOD(29,9), 29 MOD 9;
+--------------+---------+-----------+----------+
| MOD(234, 10) | 253 % 7 | MOD(29,9) | 29 MOD 9 |
+--------------+---------+-----------+----------+
|            4 |       1 |         2 |        2 |
+--------------+---------+-----------+----------+

5.RAND(), RAND(N) 取0-1之间的随机数函数
mysql> ? rand
mysql> SELECT RAND(), RAND();
+------------------+-----------------+
| RAND()           | RAND()          |
+------------------+-----------------+
| 0.77874226009356 | 0.5317868818825 |
+------------------+-----------------+

6.TRUNCATE(X,D) 返回数字X被截断为D位小数的结果
mysql> ? truncate
mysql> SELECT TRUNCATE(1.223,1), TRUNCATE(1.999,1), TRUNCATE(-1.999,2);
+-------------------+-------------------+--------------------+
| TRUNCATE(1.223,1) | TRUNCATE(1.999,1) | TRUNCATE(-1.999,2) |
+-------------------+-------------------+--------------------+
|               1.2 |               1.9 |              -1.99 |
+-------------------+-------------------+--------------------+

三、日期和时间函数
1.CURDATE() 当前日期函数
mysql> ? curdate
mysql> SELECT CURDATE(),CURDATE() + 0;
+------------+---------------+
| CURDATE()  | CURDATE() + 0 |
+------------+---------------+
| 2009-07-03 |      20090703 |
+------------+---------------+

2.CURTIME() 当前时间函数
mysql> ? curtime
mysql> SELECT CURTIME(), CURTIME() + 0;
+-----------+---------------+
| CURTIME() | CURTIME() + 0 |
+-----------+---------------+
| 12:07:08  |        120708 |
+-----------+---------------+

3.NOW() 当前日期和时间函数
mysql> ? now
mysql> SELECT NOW(), NOW() + 0;
+---------------------+----------------+
| NOW()               | NOW() + 0      |
+---------------------+----------------+
| 2009-07-03 12:07:54 | 20090703120754 |
+---------------------+----------------+

4.UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date) 日期date的时间戳
mysql> ? unix_timestamp
mysql> SELECT UNIX_TIMESTAMP(), UNIX_TIMESTAMP('1981-02-15 23:23:00');
+------------------+---------------------------------------+
| UNIX_TIMESTAMP() | UNIX_TIMESTAMP('1981-02-15 23:23:00') |
+------------------+---------------------------------------+
|       1246594366 |                             351098580 |
+------------------+---------------------------------------+

5.FROM_UNIXTIME(unix_timestamp), FROM_UNIXTIME(unix_timestamp,format) 返回时间戳的日期值（UNIX_TIMESTAMP的反函数）
mysql> ? from_unixtime
mysql> SELECT FROM_UNIXTIME(1246594135),FROM_UNIXTIME(351098580);
+---------------------------+--------------------------+
| FROM_UNIXTIME(1246594135) | FROM_UNIXTIME(351098580) |
+---------------------------+--------------------------+
| 2009-07-03 12:08:55       | 1981-02-15 23:23:00      |
+---------------------------+--------------------------+

6.WEEK(date[,mode]) 返回所给日期是一年中的第几周
mysql> ? week
mysql> SELECT WEEK('1981-02-15');
+--------------------+
| WEEK('1981-02-15') |
+--------------------+
|                  7 |
+--------------------+

7.YEAR(date)
mysql> ? year
mysql> SELECT YEAR('81-02-15');
+------------------+
| YEAR('81-02-15') |
+------------------+
|             1981 |
+------------------+

8.HOUR(time) 返回时间的小时信息
mysql> ? hour
mysql> SELECT HOUR('10:05:03');
+------------------+
| HOUR('10:05:03') |
+------------------+
|               10 |
+------------------+

9.MINUTE(time) 返回时间的分钟信息
mysql> SELECT MINUTE('98-02-03 10:05:03');
+-----------------------------+
| MINUTE('98-02-03 10:05:03') |
+-----------------------------+
|                           5 |
+-----------------------------+

10.MONTHNAME(date) 返回时间的完整月份名字
mysql> SELECT MONTHNAME('1981-02-15');
+-------------------------+
| MONTHNAME('1981-02-15') |
+-------------------------+
| February                |
+-------------------------+

11.DATE_FORMAT(date,format) 根据format格式date显示形式
mysql> ? date_format
mysql> SELECT DATE_FORMAT('1981-02-15 23:23:00', '%W %M %Y');
+------------------------------------------------+
| DATE_FORMAT('1981-02-15 23:23:00', '%W %M %Y') |
+------------------------------------------------+
| Sunday February 1981                           |
+------------------------------------------------+

12.DATE_ADD(date,INTERVAL expr type) 返回与所给日期date相差INTERVAL的时间段
mysql> select now() current, date_add(now(), INTERVAL 31 day) after31days;
+---------------------+---------------------+
| current             | after31days         |
+---------------------+---------------------+
| 2009-07-03 12:34:15 | 2009-08-03 12:34:15 |
+---------------------+---------------------+

13.DATEDIFF(expr,expr2) 计算两个日期之间相差的天数
mysql> SELECT DATEDIFF(now(),'1981-02-15 23:23:00');
+---------------------------------------+
| DATEDIFF(now(),'1981-02-15 23:23:00') |
+---------------------------------------+
|                                 10365 |
+---------------------------------------+

四、MySQL控制流程函数
首先创建演示表salary
mysql> use test;
mysql> create table salary (userid int, salary decimal(9,2));
mysql> insert into salary values (1,1000),(2,2000),(3,3000),(4,4000),(5,5000),(1,null);
mysql> select * from salary;
+--------+---------+
| userid | salary  |
+--------+---------+
|      1 | 1000.00 |
|      2 | 2000.00 |
|      3 | 3000.00 |
|      4 | 4000.00 |
|      5 | 5000.00 |
|      1 |    NULL |
+--------+---------+

1.IF(expr1,expr2,expr3)函数：如果expr1为真则返回expr2，否则返回expr3
mysql> ? IF FUNCTION
mysql> select IF( salary > 2000, 'High', 'Low') from salary;
+------------------------------------+
| if ( salary > 2000, 'High', 'Low') |
+------------------------------------+
| Low                                |
| Low                                |
| High                               |
| High                               |
| High                               |
| Low                                |
+------------------------------------+

2.IFNULL(expr1,expr2)函数：若expr1为NULL则返回expr2内容
mysql> ? IFNULL
mysql> select ifnull(salary,0) from salary;
+------------------+
| ifnull(salary,0) |
+------------------+
|          1000.00 |
|          2000.00 |
|          3000.00 |
|          4000.00 |
|          5000.00 |
|             0.00 |
+------------------+

3.NULLIF(expr1,expr2)函数：若expr1 = expr2则返回NULL，否则返回expr1
mysql> ? NULLIF
mysql> select nullif(salary,2000) from salary;
+---------------------+
| nullif(salary,2000) |
+---------------------+
|             1000.00 |
|                NULL |
|             3000.00 |
|             4000.00 |
|             5000.00 |
|                NULL |
+---------------------+

4.CASE函数
语法如下：
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END
或
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
mysql> ? CASE FUNCTION
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                                                |
+-----------------------------------------------------+

mysql> select case salary when 1000 then 'Low' when 2000 then 'Mid' else 'High' end from salary;
+-----------------------------------------------------------------------+
| case salary when 1000 then 'Low' when 2000 then 'Mid' else 'High' end |
+-----------------------------------------------------------------------+
| Low                                                                   |
| Mid                                                                   |
| High                                                                  |
| High                                                                  |
| High                                                                  |
| High                                                                  |
+-----------------------------------------------------------------------+

五、其他常用函数
1.DATABASE()函数：用于查询当前使用数据库的名字（类似Oracle的show user;）
mysql> ? database
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test       |
+------------+

2.VERSION()函数：用户查询所使用数据库的版本
mysql> ? version
mysql> SELECT VERSION();
+------------+
| VERSION()  |
+------------+
| 5.0.22-log |
+------------+

3.USER()函数：查询当前登陆用户名
mysql> SELECT USER();
+----------------+
| USER()         |
+----------------+
| root@localhost |
+----------------+

4.INET_ATON(expr)函数：查询IP地址的网络字节序表示，反函数是INET_NTOA
mysql> SELECT INET_ATON('144.194.192.183');
+------------------------------+
| INET_ATON('144.194.192.183') |
+------------------------------+
|                   2428682423 |
+------------------------------+

5.INET_NTOA(expr)函数：查询网络字节序代表的IP地址，是INET_ATON的反函数
mysql> ? inet_ntoa
mysql> SELECT INET_NTOA(2428682423);
+-----------------------+
| INET_NTOA(2428682423) |
+-----------------------+
| 144.194.192.183       |
+-----------------------+

六、小结
有事没事多请教一下帮助is a good idea. 也许这就是开源的好处，她会尽可能的考虑到您查询和参考的便利性。MySQL的函数还是很丰富的，以上实验用到的函数都是非常常用的。
这个小文儿，可以“一看了之”，只要你能想到MySQL提供的这些功能函数，check一下帮助系统，就什么都得到啦。
如果这些函数仍然没有满足您的求知欲望，敬请参考官方文档《第12章：函数和操作符》，地址为：http://dev.mysql.com/doc/refman/5.1/zh/functions.html

好运！