使用函数的方便性我就不多说了，在MySQL数据库中，函数可以使用在SELECT语句及其字句（例如WHERE、ORDER BY、HAVING等）中，也可以用在UPDATE、DELETE语句及其字句中。
一、字符串函数
注意：下面只是演示函数的作用，并未选择具体的数据库及表
1. CONCAT( s1,s2,...sn) 函数：把传入的参数连接成一个字符串
注意：任何字符串与NULL进行连接的结果都将是NULL。
mysql> select concat('aaa','bbb','ccc'),concat('aaa',null);
+---------------------------+--------------------+
| concat('aaa','bbb','ccc') | concat('aaa',null) |
+---------------------------+--------------------+
| aaabbbccc | NULL |
+---------------------------+--------------------+
1 row in set (0.00 sec)2. INSERT(str，x，y，instr)函数：将字符串str从第x位置开始，y个字符长度的字串替换为instr
mysql> select insert('beijing2008you',12,3,'me');
+------------------------------------+
| insert('beijing2008you',12,3,'me') |
+------------------------------------+
| beijing2008me |
+------------------------------------+
1 row in set (0.00 sec)3. LOWER(str)和UPPER(str)函数：把字符串转化成小写或大写
mysql> select lower('BEIJING2008'),UPPER('beijing2008');
+----------------------+----------------------+
| lower('BEIJING2008') | UPPER('beijing2008') |
+----------------------+----------------------+
| beijing2008 | BEIJING2008 |
+----------------------+----------------------+
1 row in set (0.00 sec)4. LEFT(str，x) 和 RIGHT(str，x)函数：分别返回字符串str最左/最右的x个字符
mysql> select left('beijing2008',7),left('beijing2008',null),right('beijing2008',4),right('beijing2008',null);
+-----------------------+--------------------------+------------------------+---------------------------+
| left('beijing2008',7) | left('beijing2008',null) | right('beijing2008',4) | right('beijing2008',null) |
+-----------------------+--------------------------+------------------------+---------------------------+
| beijing | NULL | 2008 | NULL |
+-----------------------+--------------------------+------------------------+---------------------------+
1 row in set (0.00 sec)注意：参数x不可省略，当为null时，返回null。
5. LPAD(str, n, pad) 和 RPAD( str, n, pad) 函数：用字符串pad对str最左边和最右边进行填充使得整个字符串长度为n
mysql> select lpad('2008',20,'beijing'),rpad('beijing',20,'2008');
+---------------------------+---------------------------+
| lpad('2008',20,'beijing') | rpad('beijing',20,'2008') |
+---------------------------+---------------------------+
| beijingbeijingbe2008 | beijing2008200820082 |
+---------------------------+---------------------------+
1 row in set (0.00 sec)6. LTRIM(str）和 RTRIM(str）函数：去掉字符串str左侧和右侧的空格
mysql> select ltrim(' |beijing'),rtrim('beijing| ');
+-----------------------+----------------------+
| ltrim(' |beijing') | rtrim('beijing| ') |
+-----------------------+----------------------+
| |beijing | beijing| |
+-----------------------+----------------------+
1 row in set (0.00 sec)7. REPEAT(str, x) 函数：将字符串str重复x次
mysql> select repeat('mysql ',3);
+--------------------+
| repeat('mysql ',3) |
+--------------------+
| mysql mysql mysql |
+--------------------+
1 row in set (0.00 sec)8. REPLACE( str, a, b）函数：用字符串b替换str中出现的所有字符串a
mysql> select replace('beijing_2010','_2010','2008');
+----------------------------------------+
| replace('beijing_2010','_2010','2008') |
+----------------------------------------+
| beijing2008 |
+----------------------------------------+
1 row in set (0.00 sec)9. STRCMP(s1, s2)函数：比较字符串s1和s2的ASCII码值的大小
如果s1比s2小返回-1，相等返回0，s1比s2大返回1 。
mysql> select strcmp('a','b'),strcmp('a','a'),strcmp('c','b');
+-----------------+-----------------+-----------------+
| strcmp('a','b') | strcmp('a','a') | strcmp('c','b') |
+-----------------+-----------------+-----------------+
| -1 | 0 | 1 |
+-----------------+-----------------+-----------------+
1 row in set (0.00 sec)10. TRIM(str)函数：去掉字符串str开头和结尾的空格
mysql> select trim(' $ beijing2008 $ ');
+---------------------------------+
| trim(' $ beijing2008 $ ') |
+---------------------------------+
| $ beijing2008 $ |
+---------------------------------+
1 row in set (0.00 sec)11. SUBSTRING（str, x, y)函数：返回字符串str中的第x位置起长度为y的字符串
mysql> select substring('beijing2008',8,4);
+------------------------------+
| substring('beijing2008',8,4) |
+------------------------------+
| 2008 |
+------------------------------+
1 row in set (0.00 sec)
该函数常用来提取字串二、数值函数
方便的处理很多数值方面的运算。
1. ABS(X)函数：返回x的绝对值
mysql> select abs(-0.8),abs(0.8);
+-----------+----------+
| abs(-0.8) | abs(0.8) |
+-----------+----------+
| 0.8 | 0.8 |
+-----------+----------+
1 row in set (0.00 sec)2. CEIL(X)函数：对数x向上取整
mysql> select ceil(-0.8),ceil(0.8);
+------------+-----------+
| ceil(-0.8) | ceil(0.8) |
+------------+-----------+
| 0 | 1 |
+------------+-----------+
1 row in set (0.00 sec)3. FLOOR(X)函数：对数x向下取整
mysql> select floor(-0.8),floor(0.8);
+-------------+------------+
| floor(-0.8) | floor(0.8) |
+-------------+------------+
| -1 | 0 |
+-------------+------------+
1 row in set (0.00 sec)4. MOD(X, Y)函数：返回x/y的模（取余数）
mysql> select mod(15,10),mod(1,11),mod(null,10),mod(1,null),mod(0,2),mod(2,0);
+------------+-----------+--------------+-------------+----------+----------+
| mod(15,10) | mod(1,11) | mod(null,10) | mod(1,null) | mod(0,2) | mod(2,0) |
+------------+-----------+--------------+-------------+----------+----------+
| 5 | 1 | NULL | NULL | 0 | NULL |
+------------+-----------+--------------+-------------+----------+----------+
1 row in set (0.00 sec)注意：x或y任意一个为null时结果为null，y为0时结果也为null。
5. RAND()函数：返回0~1内的随机数
mysql> select rand(),rand(); 每次随机的数字都不一样
+--------------------+--------------------+
| rand() | rand() |
+--------------------+--------------------+
| 0.5769799859979935 | 0.8579178544300775 |
+--------------------+--------------------+
1 row in set (0.00 sec)用该函数做相应变化可以得到指定范围内的随机数，如需要0~100内的随机整数：
mysql> select ceil(100*rand()),floor(100*rand()); 向下能取到0，向上能取到100
+------------------+-------------------+
| ceil(100*rand()) | floor(100*rand()) |
+------------------+-------------------+
| 56 | 21 |
+------------------+-------------------+
1 row in set (0.00 sec)6. ROUND(x, y) 函数：返回参数x的四舍五入的有y位小数的值
如果y省略则默认y为0，即不留小数位；这个函数通常用来统一数据格式。
mysql> select round(1.1),round(1.1,2),round(1,2);
+------------+--------------+------------+
| round(1.1) | round(1.1,2) | round(1,2) |
+------------+--------------+------------+
| 1 | 1.10 | 1 |
+------------+--------------+------------+
1 row in set (0.00 sec)注意：对于整数即使规定了小数位也不会补0，这与老版本有所区别。
7. TRUNCATE(X, Y)函数：将数字x进行截断，只保留y位小数
该函数只是截断，而round函数是四舍五入，当截断位数不足时补0。
mysql> select round(1.235,2),truncate(1.235,2),truncate(1.2,2);
+----------------+-------------------+-----------------+
| round(1.235,2) | truncate(1.235,2) | truncate(1.2,2) |
+----------------+-------------------+-----------------+
| 1.24 | 1.23 | 1.20 |
+----------------+-------------------+-----------------+
1 row in set (0.00 sec)三、日期和时间函数
时间函数可以实现很多功能，比如当前时间是多少，下个月的今天是星期几，统计前天之前的总收入等等。
1. CURDATE() 函数：返回当前时间，只有年月日
mysql> select curdate();
+------------+
| curdate() |
+------------+
| 2018-12-13 |
+------------+
1 row in set (0.00 sec)2. CURTIME()函数：返回当前时间，只有时分秒
mysql> select curtime();
+-----------+
| curtime() |
+-----------+
| 16:20:52 |
+-----------+
1 row in set (0.00 sec)3. NOW()函数：返回当前的日期和时间，包含年月日时分秒
mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2018-12-13 16:22:24 |
+---------------------+
1 row in set (0.00 sec)4. UNIX_TIMESTAMP(date)函数：返回日期date的unix时间戳
mysql> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
| 1544689715 |
+-----------------------+
1 row in set (0.00 sec)5. FROM_UNIXTIME(unixtime)函数：返回unixtime时间戳的日期值；与上面的函数互为逆操作
mysql> select from_unixtime(1544689715);
+---------------------------+
| from_unixtime(1544689715) |
+---------------------------+
| 2018-12-13 16:28:35 |
+---------------------------+
1 row in set (0.00 sec)6. WEEK(DATE)和YEAR(DATE)函数：返回所给date是哪一年，是一年中第几周
mysql> select week(now()),year(now());
+-------------+-------------+
| week(now()) | year(now()) |
+-------------+-------------+
| 49 | 2018 |
+-------------+-------------+
1 row in set (0.00 sec)7. HOUR(time)、MINUTE(time)和SECOND(time)函数：返回所给时间的小时/分钟/秒
mysql> select hour(curtime()),minute(curtime()),second(curtime());
+-----------------+-------------------+-------------------+
| hour(curtime()) | minute(curtime()) | second(curtime()) |
+-----------------+-------------------+-------------------+
| 16 | 38 | 31 |
+-----------------+-------------------+-------------------+
1 row in set (0.00 sec)8. MONTHNAME(date)和DAYNAME(date)函数：返回date的英文月份/星期名称
mysql> select monthname(now()),dayname(now());
+------------------+----------------+
| monthname(now()) | dayname(now()) |
+------------------+----------------+
| December | Thursday |
+------------------+----------------+
1 row in set (0.00 sec)9. DATE_FORMAT(date, fmt) 函数：按字符串格式fmt格式化日期date的值
关于fmt格式，有很多，硬记有点困难，下面介绍几个常用的：
mysql> select date_format(now(),'%M, %D, %Y'),date_format(now(),'%m. %d, %y');
+---------------------------------+---------------------------------+
| date_format(now(),'%M, %D, %Y') | date_format(now(),'%m. %d, %y') |
+---------------------------------+---------------------------------+
| December, 13th, 2018 | 12. 13, 18 |
+---------------------------------+---------------------------------+
1 row in set (0.00 sec)10. DATE_ADD(date，INTERVAL expr type) 函数：返回与所给日期date相差一定时间间隔的日期
INTERVAL是间隔类型关键字，expr是表达式，type是表达式对应的类型（MySQL中大概有13种）。
mysql> select now() current,date_add(now(),interval 31 day) after31days,date_add(now(),interval '1_2' year_month) after_oneyear_twomonth;
+---------------------+---------------------+------------------------+
| current | after31days | after_oneyear_twomonth |
+---------------------+---------------------+------------------------+
| 2018-12-13 17:02:17 | 2019-01-13 17:02:17 | 2020-02-13 17:02:17 |
+---------------------+---------------------+------------------------+
1 row in set (0.00 sec)当然，上面的表达式还可以添加‘-’表示它之前的某个日期时间。
11. DATEDIFF(date1，date2)函数：计算两个日期之间相差（date1-date2）的天数
mysql> select datediff('2008-08-08',now());
+------------------------------+
| datediff('2008-08-08',now()) |
+------------------------------+
| -3779 |
+------------------------------+
1 row in set (0.00 sec)四、流程函数
流程函数可以在一个SQL语句中实现条件选择，提高语句效率。
首先我们新建一个职员薪水表并插入一些数据，接下来的例子将使用该表的内容：
mysql> use test1;
Database changed
mysql> create table salary (userid int,salsry decimal(9,2));
Query OK, 0 rows affected (0.02 sec)
mysql> insert into salary values(1,1000),(2,2000),(3,3000),(4,4000),(5,5000),(1,null);
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from salary;
+--------+---------+
| userid | salsry |
+--------+---------+
| 1 | 1000.00 |
| 2 | 2000.00 |
| 3 | 3000.00 |
| 4 | 4000.00 |
| 5 | 5000.00 |
| 1 | NULL |
+--------+---------+
6 rows in set (0.00 sec)1. IF( value, t, f)函数：如果value为真返回t，否则返回f
mysql> select if(salsry > 2000,'high','low') from salary; 这里就通过一个条件将所有工资进行分类
+--------------------------------+
| if(salsry > 2000,'high','low') |
+--------------------------------+
| low |
| low |
| high |
| high |
| high |
| low |
+--------------------------------+
6 rows in set (0.00 sec)2. IFNULL(value1，value2)函数：value1不为空返回value1，否则返回value2
该函数常用来替换表中的null值。
mysql> select ifnull(salsry,0) from salary;
+------------------+
| ifnull(salsry,0) |
+------------------+
| 1000.00 |
| 2000.00 |
| 3000.00 |
| 4000.00 |
| 5000.00 |
| 0.00 |
+------------------+
6 rows in set (0.00 sec)3. CASE [expr] WHEN [value1] THEN [result1] ......ELSE [default] END函数：按条件分类
mysql> select case when salsry>2000 then 'high' else 'low' end from salary;
+--------------------------------------------------+
| case when salsry>2000 then 'high' else 'low' end |
+--------------------------------------------------+
| low |
| low |
| high |
| high |
| high |
| low |
+--------------------------------------------------+
6 rows in set (0.00 sec)
mysql> select case salsry when 1000 then 'low' when 2000 then 'mid' else 'high' end from salary;
+-----------------------------------------------------------------------+
| case salsry when 1000 then 'low' when 2000 then 'mid' else 'high' end |
+-----------------------------------------------------------------------+
| low |
| mid |
| high |
| high |
| high |
| high |
+-----------------------------------------------------------------------+
6 rows in set (0.00 sec)用该函数可以实现多分类。
五、其它常用函数
1. DATABASE()函数：返回当前数据库名
mysql> select database();
+------------+
| database() |
+------------+
| test1 |
+------------+
1 row in set (0.00 sec)2. VERDION()函数：返回当前数据库版本
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.17-log |
+------------+
1 row in set (0.00 sec)3. USER()函数：返回当前登陆用户名
mysql> select user();
+----------------+
| user() |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)4. INET_ATON(IP)函数：返回IP地址的网络字节序表示
mysql> select inet_aton('192.168.1.1');
+--------------------------+
| inet_aton('192.168.1.1') |
+--------------------------+
| 3232235777 |
+--------------------------+
1 row in set (0.00 sec)5. INET_NTOA(num)函数：返回网络字节序代表的IP地址
mysql> select inet_ntoa(3232235777)
-> ;
+-----------------------+
| inet_ntoa(3232235777) |
+-----------------------+
| 192.168.1.1 |
+-----------------------+
1 row in set (0.00 sec)上面两个函数的主要作用是对IP的相互转化，它可以实现ip地址的比较，比如某个表里存了很多IP地址，你如要想要找到ip在‘192.168.1.3’和‘192.168.1.20’之间一共有多少个IP地址时，如果直接用ip比较，那么它会一个字符一个字符的比较，比较到3和2时由于3大于2导致这两个ip之间是没有其它ip的，这显然不对，因此需要转为网络字节序来进行比较：
select * from t where inet_aton(ip) >= inet_aton('192.168.1.3') and inet_aton(ip) <= inet_aton('192.168.1.20');
这样就可以将表t中ip字段在指定值之间的所有IP查找出来。
6. PASSWORD(str)函数：返回字符串str的加密版本，一个41位长的字符串
注意：此函数只用来加密系统用户的密码，表内数据的加密需要使用MD5等函数。
mysql> select password('123456');
+-------------------------------------------+
| password('123456') |
+-------------------------------------------+
| *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)7. MD5(str)函数：返回字符串str的MD5值，可对数据加密
mysql> select MD5('123456');
+----------------------------------+
| MD5('123456') |
+----------------------------------+
| e10adc3949ba59abbe56e057f20f883e |
+----------------------------------+
1 row in set (0.00 sec)MySQL中还有很多非常有用的内建函数，后面遇到再补充，平时在使用时注意查看MySQL内是否有相应函数，使用它们可以极大提高效率。