一个字符串，一个数组，是我认为在编程里面最经常用到的数据类型。
MySQL提供了一套为字符串操作而设计的函数，其功能还是非常强大的。
其实呢，我说的这些函数你不一定都要记，我是背不下来的，天生懒人。否则我就去学文学了，不过这是一个遗憾就不说了。
但是，你要知道有这么一些函数，它可以让你在使用MySQL操作字符串的时候得心应手，事半功倍。
闲话不说了，继续学习，MySQL那些你不知道的字符串函数。
LENGTH()函数可以获得一个字符串的长度：
select length('abcdefg'),length('0123456789');
/*
+-------------------+----------------------+
| length('abcdefg') | length('0123456789') |
+-------------------+----------------------+
| 7 | 10 |
+-------------------+----------------------+
1 row in set (0.00 sec)
*/
/*
+-------------------+----------------------+
| length('abcdefg') | length('0123456789') |
+-------------------+----------------------+
| 7 | 10 |
+-------------------+----------------------+
1 row in set (0.00 sec)
*/
使用RPAD()或者LPAD()函数可以分别从右边或者左边对字符串进行填补：
一般，字符串经常用空格进行填补（就是所谓的补齐），MySQL可以通过给RPAD()和LPAD()函数中指定一个自定义值作为第三个参数对字符串进行填补。
select rpad('simaopig',17,''),rpad('simaopig',17,'@'),lpad('simaopig',17,''),lpad('simaopig',17,'@');
/*
+------------------------+-------------------------+------------------------+-------------------------+
| rpad('simaopig',17,'') | rpad('simaopig',17,'@') | lpad('simaopig',17,'') | lpad('simaopig',17,'@') |
+------------------------+-------------------------+------------------------+-------------------------+
| NULL | simaopig@@@@@@@@@ | NULL | @@@@@@@@@simaopig |
+------------------------+-------------------------+------------------------+-------------------------+
1 row in set (0.00 sec)
*/
select rpad('simaopig',17,' '),rpad('simaopig',17,'@'),lpad('simaopig',17,' '),lpad('simaopig',17,'@');
/*
+-------------------------+-------------------------+-------------------------+-------------------------+
| rpad('simaopig',17,' ') | rpad('simaopig',17,'@') | lpad('simaopig',17,' ') | lpad('simaopig',17,'@') |
+-------------------------+-------------------------+-------------------------+-------------------------+
| simaopig | simaopig@@@@@@@@@ | simaopig | @@@@@@@@@simaopig |
+-------------------------+-------------------------+-------------------------+-------------------------+
1 row in set (0.00 sec)
*/
/*
+------------------------+-------------------------+------------------------+-------------------------+
| rpad('simaopig',17,'') | rpad('simaopig',17,'@') | lpad('simaopig',17,'') | lpad('simaopig',17,'@') |
+------------------------+-------------------------+------------------------+-------------------------+
| NULL | simaopig@@@@@@@@@ | NULL | @@@@@@@@@simaopig |
+------------------------+-------------------------+------------------------+-------------------------+
1 row in set (0.00 sec)
*/
select rpad('simaopig',17,' '),rpad('simaopig',17,'@'),lpad('simaopig',17,' '),lpad('simaopig',17,'@');
/*
+-------------------------+-------------------------+-------------------------+-------------------------+
| rpad('simaopig',17,' ') | rpad('simaopig',17,'@') | lpad('simaopig',17,' ') | lpad('simaopig',17,'@') |
+-------------------------+-------------------------+-------------------------+-------------------------+
| simaopig | simaopig@@@@@@@@@ | simaopig | @@@@@@@@@simaopig |
+-------------------------+-------------------------+-------------------------+-------------------------+
1 row in set (0.00 sec)
*/
LTRIM()和RTRIM()函数产生与填补相反的效果，去除字符串的首和尾字符：
select ltrim('simaopig '),ltrim(' simaopig'),rtrim('simaopig '),rtrim(' simaopig'),ltrim(rtrim(' simaopig '));
/*
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
| ltrim('simaopig ') | ltrim(' simaopig') | rtrim('simaopig ') | rtrim(' simaopig') | ltrim(rtrim(' simaopig ')) |
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
| simaopig | simaopig | simaopig | simaopig | simaopig |
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
1 row in set (0.00 sec)
*/
/*
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
| ltrim('simaopig ') | ltrim(' simaopig') | rtrim('simaopig ') | rtrim(' simaopig') | ltrim(rtrim(' simaopig ')) |
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
| simaopig | simaopig | simaopig | simaopig | simaopig |
+----------------------+---------------------------+----------------------+--------------------------+----------------------------------+
1 row in set (0.00 sec)
*/
即然谈到了“空格”，那么把书中的例子再抄一遍，那就是MySQL提供了一个函数，这个函数(SPACE())的功能是只返回空格
select space(1),space(4),length(space(1)),length(space(4));
/*
+----------+----------+------------------+------------------+
| space(1) | space(4) | length(space(1)) | length(space(4)) |
+----------+----------+------------------+------------------+
| | | 1 | 4 |
+----------+----------+------------------+------------------+
1 row in set (0.00 sec)
*/
/*
+----------+----------+------------------+------------------+
| space(1) | space(4) | length(space(1)) | length(space(4)) |
+----------+----------+------------------+------------------+
| | | 1 | 4 |
+----------+----------+------------------+------------------+
1 row in set (0.00 sec)
*/
TRIM()函数可以指定去除格式，并且可以指定是去除左边还是右边？或者去除其它字符（不发局限于空格)：
其实啊，就是把RTRIM()和LTRIM()的活都做了，而且做的好，可以说TRIM()和RTRIM()是TRIM()的子集。。。
select trim(' simaopig '),length(trim(' simaopig ')) as len;
/*
+--------------------+-----+
| trim(' simaopig ') | len |
+--------------------+-----+
| simaopig | 8 |
+--------------------+-----+
1 row in set (0.00 sec)
*/
select trim(leading '!' from '!!!simaopig!!!') as trim_leading,trim(trailing '!' from '!!!simaopig!!!') as trim_trailing,trim(both '!' from '!!!simaopig!!!') as trim_both;
/*
+--------------+---------------+-----------+
| trim_leading | trim_trailing | trim_both |
+--------------+---------------+-----------+
| simaopig!!! | !!!simaopig | simaopig |
+--------------+---------------+-----------+
1 row in set (0.00 sec)
*/
/*
+--------------------+-----+
| trim(' simaopig ') | len |
+--------------------+-----+
| simaopig | 8 |
+--------------------+-----+
1 row in set (0.00 sec)
*/
select trim(leading '!' from '!!!simaopig!!!') as trim_leading,trim(trailing '!' from '!!!simaopig!!!') as trim_trailing,trim(both '!' from '!!!simaopig!!!') as trim_both;
/*
+--------------+---------------+-----------+
| trim_leading | trim_trailing | trim_both |
+--------------+---------------+-----------+
| simaopig!!! | !!!simaopig | simaopig |
+--------------+---------------+-----------+
1 row in set (0.00 sec)
*/
LEFT()和RIGHT()函数可以获得字符串的指定部分，它将从字符串的左边或右边返回字符：
select left('simaopig',5) as left_five,right('simaopig',5) as right_five,left(right('simaopig',7),5) as midd_five;
/*
+-----------+------------+-----------+
| left_five | right_five | midd_five |
+-----------+------------+-----------+
| simao | aopig | imaop |
+-----------+------------+-----------+
1 row in set (0.00 sec)
*/
/*
+-----------+------------+-----------+
| left_five | right_five | midd_five |
+-----------+------------+-----------+
| simao | aopig | imaop |
+-----------+------------+-----------+
1 row in set (0.00 sec)
*/
SUBSTRING()函数，允许指定一个字符串的长度和起始位置，也是获取子字符串：
注意哈，这里的MySQL的字符串起始位置是1，不是零哈。
select substring('simaopig',2,3);
/*
+---------------------------+
| substring('simaopig',2,3) |
+---------------------------+
| ima |
+---------------------------+
1 row in set (0.00 sec)
*/
/*
+---------------------------+
| substring('simaopig',2,3) |
+---------------------------+
| ima |
+---------------------------+
1 row in set (0.00 sec)
*/
CONCAT()函数使其内提供的参数连接起来：
如果CONCAT()的参数是空的，其结果也是空的（NULL）。
select concat('welcome','to','小小子'),concat('simaopig',NULL);
/*
+------------------------------------+-------------------------+
| concat('welcome','to','小小子') | concat('simaopig',NULL) |
+------------------------------------+-------------------------+
| welcometo小小子 | NULL |
+------------------------------------+-------------------------+
1 row in set (0.00 sec)
*/
/*
+------------------------------------+-------------------------+
| concat('welcome','to','小小子') | concat('simaopig',NULL) |
+------------------------------------+-------------------------+
| welcometo小小子 | NULL |
+------------------------------------+-------------------------+
1 row in set (0.00 sec)
*/
CONCAT_WS()函数与CONCAT()函数基本相同，只是其还可以提供一个参数作为连接后字符串的分隔符：
select concat_ws(',','welcome','to','xiaoxiaozi.com') as con_wel,concat_ws(' ','Simaopig','Love','Yatou') as con_love;
/*
+---------------------------+---------------------+
| con_wel | con_love |
+---------------------------+---------------------+
| welcome,to,xiaoxiaozi.com | Simaopig Love Yatou |
+---------------------------+---------------------+
1 row in set (0.00 sec)
*/
/*
+---------------------------+---------------------+
| con_wel | con_love |
+---------------------------+---------------------+
| welcome,to,xiaoxiaozi.com | Simaopig Love Yatou |
+---------------------------+---------------------+
1 row in set (0.00 sec)
*/
使用LOCATE()函数检验一个字符串是否包含所要检测的字符串，如果存在，返回其第一次出现的位置：
今天，第二次强调。MySQL字符串起始位置为1，不是零。
select locate('pig','simaopig'),locate('','simaopig'),locate(' ','simaopig'),locate(' ',' simaopig ');
/*
+--------------------------+-----------------------+------------------------+--------------------------+
| locate('pig','simaopig') | locate('','simaopig') | locate(' ','simaopig') | locate(' ',' simaopig ') |
+--------------------------+-----------------------+------------------------+--------------------------+
| 6 | 1 | 0 | 1 |
+--------------------------+-----------------------+------------------------+--------------------------+
1 row in set (0.00 sec)
*/
/*
+--------------------------+-----------------------+------------------------+--------------------------+
| locate('pig','simaopig') | locate('','simaopig') | locate(' ','simaopig') | locate(' ',' simaopig ') |
+--------------------------+-----------------------+------------------------+--------------------------+
| 6 | 1 | 0 | 1 |
+--------------------------+-----------------------+------------------------+--------------------------+
1 row in set (0.00 sec)
*/
FIND_IN_SET()函数是在一个大集合中查找指定的字符串：
在MySQL中看到set就应该想到集合！！！
select find_in_set('simaopig','My,Name,Is,Simaopig,You,Can,Call,Me,simaopig,Too') as string_locate;
/*
+---------------+
| string_locate |
+---------------+
| 4 |
+---------------+
1 row in set (0.00 sec)
*/
/*
+---------------+
| string_locate |
+---------------+
| 4 |
+---------------+
1 row in set (0.00 sec)
*/
哈，看到了吧，其返回的是该字符串是集合的第几个元素，嗯。
你还问为啥大写的也返回了？呃，因为我没有用binary啊。
STRCMP()函数比较两个字符串是否相同，相同返回0，如果第一个大于第二个返回1，否则返回-1：
呃，同样的，不加binary的话，字符串比较是不区分大小写的。
select strcmp('a','b'),strcmp('b','a'),strcmp('a','a'),strcmp('a','A'),strcmp('a','B');
/*
+-----------------+-----------------+-----------------+-----------------+-----------------+
| strcmp('a','b') | strcmp('b','a') | strcmp('a','a') | strcmp('a','A') | strcmp('a','B') |
+-----------------+-----------------+-----------------+-----------------+-----------------+
| -1 | 1 | 0 | 0 | -1 |
+-----------------+-----------------+-----------------+-----------------+-----------------+
1 row in set (0.00 sec)
*/
/*
+-----------------+-----------------+-----------------+-----------------+-----------------+
| strcmp('a','b') | strcmp('b','a') | strcmp('a','a') | strcmp('a','A') | strcmp('a','B') |
+-----------------+-----------------+-----------------+-----------------+-----------------+
| -1 | 1 | 0 | 0 | -1 |
+-----------------+-----------------+-----------------+-----------------+-----------------+
1 row in set (0.00 sec)
*/
REPLACE()函数和在别的编程语言里的用法一致，替换功能：
select replace('I am simaopig','simaopig','xiaoxiaozi');
/*
+--------------------------------------------------+
| replace('I am simaopig','simaopig','xiaoxiaozi') |
+--------------------------------------------------+
| I am xiaoxiaozi |
+--------------------------------------------------+
1 row in set (0.00 sec)
*/
/*
+--------------------------------------------------+
| replace('I am simaopig','simaopig','xiaoxiaozi') |
+--------------------------------------------------+
| I am xiaoxiaozi |
+--------------------------------------------------+
1 row in set (0.00 sec)
*/
INSSERT()函数可以把一个字符串的指定部分（定义了起始位置和长度）替换为一个新值：
select insert('I am simaopig',6,8,'xiaoxiaozi');
/*
+------------------------------------------+
| insert('I am simaopig',6,8,'xiaoxiaozi') |
+------------------------------------------+
| I am xiaoxiaozi |
+------------------------------------------+
1 row in set (0.00 sec)
*/
/*
+------------------------------------------+
| insert('I am simaopig',6,8,'xiaoxiaozi') |
+------------------------------------------+
| I am xiaoxiaozi |
+------------------------------------------+
1 row in set (0.00 sec)
*/
REPEAT()函数用于重复字符串操作：
例如：输出十遍’xiaoxiaozi’
select repeat('xiaoxiaozi',10);
/*
+------------------------------------------------------------------------------------------------------+
| repeat('xiaoxiaozi',10) |
+------------------------------------------------------------------------------------------------------+
| xiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozi |
+------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
*/
/*
+------------------------------------------------------------------------------------------------------+
| repeat('xiaoxiaozi',10) |
+------------------------------------------------------------------------------------------------------+
| xiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozixiaoxiaozi |
+------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
*/
REVERSE()函数颠倒字符串：
select reverse('simaopig'),reverse(repeat('hello ',6));
/*
+---------------------+--------------------------------------+
| reverse('simaopig') | reverse(repeat('hello ',6)) |
+---------------------+--------------------------------------+
| gipoamis | olleh olleh olleh olleh olleh olleh |
+---------------------+--------------------------------------+
1 row in set (0.00 sec)
*/
/*
+---------------------+--------------------------------------+
| reverse('simaopig') | reverse(repeat('hello ',6)) |
+---------------------+--------------------------------------+
| gipoamis | olleh olleh olleh olleh olleh olleh |
+---------------------+--------------------------------------+
1 row in set (0.00 sec)
*/
UCASE()和LCASE()将字符串分别转化为大写和小写：
select ucase('simaopig'),ucase('Simaopig'),lcase('SIMAOPIG'),lcase('simaopiG');
/*
+-------------------+-------------------+-------------------+-------------------+
| ucase('simaopig') | ucase('Simaopig') | lcase('SIMAOPIG') | lcase('simaopiG') |
+-------------------+-------------------+-------------------+-------------------+
| SIMAOPIG | SIMAOPIG | simaopig | simaopig |
+-------------------+-------------------+-------------------+-------------------+
1 row in set (0.00 sec)
*/
/*
+-------------------+-------------------+-------------------+-------------------+
| ucase('simaopig') | ucase('Simaopig') | lcase('SIMAOPIG') | lcase('simaopiG') |
+-------------------+-------------------+-------------------+-------------------+
| SIMAOPIG | SIMAOPIG | simaopig | simaopig |
+-------------------+-------------------+-------------------+-------------------+
1 row in set (0.00 sec)
*/
ASCII()函数返回指定字符的ASCII码：
select ascii('y'),ascii('Simaopig'),ascii('simaopig');
/*
+------------+-------------------+-------------------+
| ascii('y') | ascii('Simaopig') | ascii('simaopig') |
+------------+-------------------+-------------------+
| 121 | 83 | 115 |
+------------+-------------------+-------------------+
1 row in set (0.00 sec)
*/
/*
+------------+-------------------+-------------------+
| ascii('y') | ascii('Simaopig') | ascii('simaopig') |
+------------+-------------------+-------------------+
| 121 | 83 | 115 |
+------------+-------------------+-------------------+
1 row in set (0.00 sec)
*/
ORD()函数返回指定字符的数字编码，经常用于替代ASCII()：
呃，我咋感觉其返回值与ASCII()一模一样。PHP也有这个函数，貌似 。
select ord('y'),ord('Simaopig'),ord('simaopig');
/*
+----------+-----------------+-----------------+
| ord('y') | ord('Simaopig') | ord('simaopig') |
+----------+-----------------+-----------------+
| 121 | 83 | 115 |
+----------+-----------------+-----------------+
1 row in set (0.00 sec)
*/
/*
+----------+-----------------+-----------------+
| ord('y') | ord('Simaopig') | ord('simaopig') |
+----------+-----------------+-----------------+
| 121 | 83 | 115 |
+----------+-----------------+-----------------+
1 row in set (0.00 sec)
*/
呜。。今天这篇日志又写这么长，没办法，不想把这些函数分家，所以一起写吧。
累死我了，啊啊啊。。 
