MySQL字符串函数
字符串函数
length()
求字符串的长度
注意 若为null时,如果不加单引号,则返回null,若加单引号,则返回字符的长度.
mysql> select length(@de)
-> ;
+-------------+
| length(@de) |
+-------------+
| 19 |
+-------------+
1 row in set (0.00 sec)
mysql> select @de
-> ;
+---------------------+
| @de |
+---------------------+
| 2022-06-02 14:49:27 |
+---------------------+
1 row in set (0.00 sec)
mysql> select length('hello every');
+-----------------------+
| length('hello every') |
+-----------------------+
| 11 |
+-----------------------+
1 row in set (0.00 sec)
concat()
合并字符串函数，返回结果为连接参数产生的字符串，参数可以使一个或多个
参数
若有任何一个参数为 NULL，则返回值为 NULL。若所有参数均为非二进制字符串，则结果为非二进制字符串。若自变量中含有任一二进制字符串，则结果为一个二进制字符串。
实例
mysql> select concat('hello',' world');
+---------------------------+
| concat('hello',' world') |
+---------------------------+
| hello world |
+---------------------------+
1 row in set (0.00 sec)
mysql> select concat('hello',3,'i world');
+------------------------------+
| concat('hello',3,'i world') |
+------------------------------+
| hello3i world |
+------------------------------+
1 row in set (0.00 sec)
insert()
替换字符串
参数
insert(s1,x,len,s2)
替换字符串函数 INSERT(s1，x，len，s2) 返回字符串 s1，子字符串起始于 x 位置，并且用 len 个字符长的字符串代替 s2
若 x 超过字符串长度，则返回值为原始字符串。假如 len 的长度大于其他字符串的长度，则从位置 x 开始替换。若任何一个参数为 NULL，则返回值为 NULL。
mysql> select insert('hello',2,2,'world') as c1;
+----------+
| c1 |
+----------+
| hworldlo |
+----------+
1 row in set (0.00 sec)
mysql> select insert('hello',2,6,'world') as c2;
+--------+
| c2 |
+--------+
| hworld |
+--------+
1 row in set (0.00 sec)
mysql> select insert('hello',2,'world') as c3;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') as c3' at line 1
mysql> select insert('hello',-1,3,'world') as c4;
+-------+
| c4 |
+-------+
| hello |
+-------+
1 row in set (0.00 sec)
mysql> select insert('hello',-4,3,'world') as c5;
+-------+
| c5 |
+-------+
| hello |
+-------+
1 row in set (0.00 sec)
replace()
替换字符串
参数
replace(s,s1,s2)
使用字符串 s2 替换字符串 s 中所有的字符串 s1
mysql> select replace('hello world','o','k');
+--------------------------------+
| replace('hello world','o','k') |
+--------------------------------+
| hellk wkrld |
+--------------------------------+
1 row in set (0.00 sec)
mysql> select replace('hello world','o','null');
+-----------------------------------+
| replace('hello world','o','null') |
+-----------------------------------+
| hellnull wnullrld |
+-----------------------------------+
1 row in set (0.00 sec)
mysql> select replace('hello world','o',null);
+---------------------------------+
| replace('hello world','o',null) |
+---------------------------------+
| NULL |
+---------------------------------+
1 row in set (0.00 sec)
substr()
截取字符串
参数
SUBSTRING(s，n，len)
-
其中len可以省略,当省略时,表示从指定的截取位置开始,至到最后。
-
从字符串 s 返回一个长度同 len 字符相同的子字符串，起始于位置 n。
-
当n 为负值,则子字符串的位置起始于字符串结尾的第 n 个字符，即倒数第 n 个字符，而不是字符串的开头位置。
mysql> select substr('hello world',2,4);
+---------------------------+
| substr('hello world',2,4) |
+---------------------------+
| ello |
+---------------------------+
1 row in set (0.00 sec)
mysql> select substr('hello world',2);
+-------------------------+
| substr('hello world',2) |
+-------------------------+
| ello world |
+-------------------------+
1 row in set (0.00 sec)
mysql> select substr('hello world',-1,4);
+----------------------------+
| substr('hello world',-1,4) |
+----------------------------+
| d |
+----------------------------+
1 row in set (0.00 sec)
mysql> select substr('hello world',2,10);
+----------------------------+
| substr('hello world',2,10) |
+----------------------------+
| ello world |
+----------------------------+
1 row in set (0.00 sec)
strm()
去除字符串两边的空格
mysql> select concat('[[',trim(' hello world '),']]');
+---------------------------------------------+
| concat('[[',trim(' hello world '),']]') |
+---------------------------------------------+
| [[hello world]] |
+---------------------------------------------+
1 row in set (0.00 sec)
reverse()
字符串反转
mysql> select reverse('hello');
+------------------+
| reverse('hello') |
+------------------+
| olleh |
+------------------+
1 row in set (0.00 sec
left(s1,len)
从字符串左侧开始,截取字符串
当为负数时,返回空
mysql> select left('hello',4)
-> ;
+-----------------+
| left('hello',4) |
+-----------------+
| hell |
+-----------------+
1 row in set (0.00 sec)
mysql> select left('hello',6);
+-----------------+
| left('hello',6) |
+-----------------+
| hello |
+-----------------+
1 row in set (0.00 sec)
mysql> select left('hello',-1);
+------------------+
| left('hello',-1) |
+------------------+
| |
+------------------+
1 row in set (0.00 sec)
right(s1,len)
从右侧开始截取字符串
mysql> select right('hello',1)
-> ;
+------------------+
| right('hello',1) |
+------------------+
| o |
+------------------+
1 row in set (0.00 sec)
mysql> select right('hello',6)
-> ;
+------------------+
| right('hello',6) |
+------------------+
| hello |
+------------------+
1 row in set (0.00 sec)
mysql> select right('hello',-1);
+-------------------+
| right('hello',-1) |
+-------------------+
| |
+-------------------+
1 row in set (0.00 sec)
lower()
将字符串全部转化为小写
mysql> select lower('HELLO WoRlD');
+----------------------+
| lower('HELLO WoRlD') |
+----------------------+
| hello world |
+----------------------+
1 row in set (0.00 sec)
upper()
将字符创全部转化为大写
mysql> select upper('hello world');
+----------------------+
| upper('hello world') |
+----------------------+
| HELLO WORLD |
+----------------------+
1 row in set (0.00 sec)