MySQL函数学习笔记二：字符函数_MySQL

1. 计算字符串字符数和字符串长度 - CHAR_LENGTH(s)

CHAR_LENGTH(str): 返回str所包含的字符个数。

mysql> select CHAR_LENGTH('MySQL');

+----------------------+

| CHAR_LENGTH('MySQL') |

+----------------------+

| 5 |

+----------------------+

2. 合并字符 - CONCAT(s1,s2,...)　与　CONCAT_WS(x,s1,s2,...)

CONCAT(s1,s2,...):回结果为连接参数产生的字符串，或许有一个或者多个参数。如果有任何一个返回值为NULL,　则返回值为NULL.

mysql> select CONCAT('MySQL',' ','5.5',' ',NULL,'Function');

+-----------------------------------------------+

| CONCAT('MySQL',' ','5.5',' ',NULL,'Function') |

+-----------------------------------------------+

| NULL |

+-----------------------------------------------+

CONCAT_WS(x,s1,s2,...): 代表CONCAT With Separator, 是CONCAT的特殊形式。第一个参数Ｘ是其它参数的分隔符，分隔符的位置在要连接的字符串之间。分隔符可以是一个字符串，也可是其他参数。如果分隔符为NULL,则结果为NULL。

mysql> select CONCAT_WS('.','David','Tian'), CONCAT_WS(NULL,'MySQL','5.5');

+-------------------------------+-------------------------------+

| CONCAT_WS('.','David','Tian') | CONCAT_WS(NULL,'MySQL','5.5') |

+-------------------------------+-------------------------------+

| David.Tian | NULL |

+-------------------------------+-------------------------------+

3. 替换字符串函数 - INSERT(s1, x, len, s2)

INSERT(s1,x,len,s2):返回字符串s1, 其子字符串起始于x位置和被字符串s2取代的len字符。如果x超过字符串长度，则返回值为原始字符串。假如len的长度大于其它字符串的长度，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('Softtekian',2,4,'!@#$') AS c1,

-> INSERT('Softtekian',-1,4,'@@@@') as c2,

-> INSERT('Softtekian',3,100,'$$') as c3,

-> INSERT('Softtekian',2,4,'%@') as c4;

+------------+------------+------+----------+

| c1 | c2 | c3 | c4 |

+------------+------------+------+----------+

| S!@#$ekian | Softtekian | So$$ | S%@ekian |

+------------+------------+------+----------+

4. 字母大小写转换函数- LOWER(s), LCASE(s), UPPER(s), UCASE(s)

LOWER(str)和LCASE(str):将字符串str中的字母全部转换成小写字母。

mysql> select LOWER('MySQL and Oracle ASM') as c1, LCASE('Database Administrator') as c2;

+----------------------+------------------------+

| c1 | c2 |

+----------------------+------------------------+

| mysql and oracle asm | database administrator |

+----------------------+------------------------+ 

UPPER(str)和UCASE(str):可以将字符串str中的字母全部转换成大写字母。

mysql> select UPPER('sunshine.ma') c1,UCASE('Sunshine.Ma') c2;

+-------------+-------------+

| c1 | c2 |

+-------------+-------------+

| SUNSHINE.MA | SUNSHINE.MA |

+-------------+-------------+

5. 获取指定长度字符串：LEFT(s,n), RIGHT(s,n)

LEFT(s,n): 返回字符串s开始最左边n个字符。

mysql> select LEFT('this is a testing email',7) as c1;

+---------+

| c1 |

+---------+

| this is |

+---------+

RIGHT(s,n):返回字符串str最右边n个字符。

mysql> select RIGHT('this is a testing email',7) as c1;

+---------+

| c1 |

+---------+

| g email |

+---------+

6. 填充字符串函数：LPAD(s1, len, s2), RPAD(s1, len, s2)

LPAD(s1, len, s2): 返回字符串s1，其左边由字符串s2填补到len字符长度。假如s1的长度大于len, 则返回值缩短至len字符。

mysql> select LPAD('Hello',4,'%%') as c1, LPAD('Hello',10,'*') as c2;

+------+------------+

| c1 | c2 |

+------+------------+

| Hell | *****Hello |

+------+------------+ 

RPAD(s1, len, s2): 返回字符串s1,　其右边被字符串s2填补至len字符串s1的长度大于len, 则返值被缩短到len字符长度。

mysql> select RPAD('Hello',4,'%') as c1, RPAD('Hello',10,'*') as c2;

+------+------------+

| c1 | c2 |

+------+------------+

| Hell | Hello***** |

+------+------------+ 

7. 删除空格字符串函数：LTRIM(s), RTRIM(s), TRIM(s)

LTRIM(s): 返回字符串s,字符串左侧空格字符被删除。
mysql> select LTRIM(' book ') as c1;

+---------+

| c1 |

+---------+

| book |

+---------+

RTRIM(s): 返回字符串s,字符串右侧空格字符被删除。
mysql> select RTRIM(' book ') as c1;

+---------+

| c1 |

+---------+

| book |

+---------+

TRIM(s): 返回字符串s,字符串两侧空格字符被删除。

mysql> select TRIM(' book ') as c1;
+------+
| c1 |
+------+
| book |
+------+
 

8. 删除指定字符串的函数：TRIM(s1 FROM s)

TRIM(s1 FROM s): 删除字符串s中两端所有的子字符串s1。s1为可选项，在未指定情况下，删除空格。

mysql> select TRIM(' boook ') as c1, TRIM('xy' FROM 'xyxyDxyDxyxy') as c2;

+-------+------+

| c1 | c2 |

+-------+------+

| boook | DxyD |

+-------+------+

9. 重复生成字符串的函数：REPEAT(s,n)

REPEAT(s,n): 返回一个由重复的字符串s组成的字符串，字符串s的数目等于n。若n

mysql> select REPEAT('abc',3) as c1, REPEAT('abc',-1) as c2, REPEAT('abc',NULL) as c3;

+-----------+------+------+

| c1 | c2 | c3 |

+-----------+------+------+

| abcabcabc | | NULL |

+-----------+------+------+

10. 空格函数：SPACE(n)

SPACE(n):返回一个由n个空格组成的字符串。

mysql> select CONCAT('(',SPACE(6),')') AS c1, CHAR_LENGTH(SPACE(6)) AS C2;

+----------+----+

| c1 | C2 |

+----------+----+

| ( ) | 6 |

11. 替换函数：REPLACE(s,s1,s2)

REPLACE(s,s1,s2):使用字符串s2替代字符串s中所有的字符串s1。

mysql> select REPLACE('xxx.mysql.com','x','w') as c1;

+---------------+

| c1 |

+---------------+

| www.mysql.com |

12. 比较字符串大小函数：STRCMP(s1,s2)

STRCMP(s1,s2):若所有的字符串均相同，则返回0；若根据当前分类次序，第一个参数小于第二个，则返回-1,其它情况返回1。

mysql> select STRCMP('txt','txta') as c1, STRCMP('txta','txt') as c2, STRCMP('txt','txt') as c3;

+----+----+----+

| c1 | c2 | c3 |

+----+----+----+

| -1 | 1 | 0 |

+----+----+----+

13. 字符串截取函数：SUBSTRING(s,n,len), MID(s,n,len)

SUBSTRING(s,n,len):从字符串s返回一个长度为len的子字符串，起始位置为n。若n为负数，则子字符串的位置起始于字符串结尾的n个字符，即倒数第n个字符。若len省略，则取至结尾。

mysql> select SUBSTRING('breaskfast',5) as c1,

-> SUBSTRING('breaskfast',5,3) as c2,

-> SUBSTRING('breakfast',-3) as c3,

-> SUBSTRING('breakfast',-5,3) as c4;

+--------+-----+-----+-----+

| c1 | c2 | c3 | c4 |

+--------+-----+-----+-----+

| skfast | skf | ast | kfa |

+--------+-----+-----+-----+

MID(s,n,len): 与SUBSTRING(s,n,len)作用相同。

mysql> select MID('breaskfast',5) as c1, MID('breaskfast',5,3) as c2, MID('breakfast',-3) as c3, MID('breakfast',-5,3) as c4;+--------+-----+-----+-----+

| c1 | c2 | c3 | c4 |

+--------+-----+-----+-----+

| skfast | skf | ast | kfa |

+--------+-----+-----+-----+

14. 匹配子串开始位置函数：LOCATE(s1,s2), POSITION(s1 IN s2), INSTR(s2,s1)

LOCATE(s1,s2): 返回子字符串s1在字符串s2中的开始位置。

POSITION(s1 IN s2): 返回子字符串s1在字符串s2中的开始位置。

INSTR(s2,s1):返回子字符串s1在字符串s2中的开始位置。

mysql> select LOCATE('ball','football') c1,

-> POSITION('ball' IN 'football') c2,

-> INSTR('football','ball') c3;

+----+----+----+

| c1 | c2 | c3 |

+----+----+----+

| 5 | 5 | 5 |

+----+----+----+

15. 字符串逆序函数：REVERSE(s)

REVERSE(s): 将字符串s反转，返回的字符串的顺序和s字符串顺序相反。

mysql> select REVERSE('I love you') as c1;

+------------+

| c1 |

+------------+

| uoy evol I |

+------------+

16. 返回指定位置的字符串函数：ELT(n,s1,s2,s3,...,Sn)

ELT(n,s1,s2,s3,...,Sn): 若n=1，则返回字符串S1，若n=2,则返回字符串S2,依此类推。若n小于1或大于参数的数目，则返回值为NULL。

mysql> select ELT(3,'1st','2nd','3rd') as c1, ELT(3,'oracle','MySQL') as c2;

+------+------+

| c1 | c2 |

+------+------+

| 3rd | NULL |

+------+------+

17. 返回指定字符串位置的函数：FIELD(s,s1,s2,...)

FIELD(s,s1,s2,...):返回字符串s在列表s1,s2,...中第一次出现的位置，在找不到s的情况下，返回值为0。如果s为NULL，则返回值为0，原因是NULL不能同任何值进行同等比较。

mysql> select FIELD('Hi','hihi','Hey','Hi','bas','ciao') as c1, FIELD('Hi','Hey','Lo','Hilo','foo') as c2;

+----+----+

| c1 | c2 |

+----+----+

| 3 | 0 |

+----+----+

18. 返回子串位置的函数：FIND_IN_SET(s1,s2)

FIND_IN_SET(s1,s2): 返回字符串s1在字符串s2中出现的位置，字符串列表是一个由多个逗号“,”分开的字符串组成的列表。如果s1不在s2中或s2为空字符串，则返回0。如果任何一个参数为NULL，则返回值为NULL。S1中不能包含一个逗号“,”。

mysql> select FIND_IN_SET('Hi','hihi,Hey,Hi,bas') as c1;

+----+

| c1 |

+----+

| 3 |

+----+

19. 选取字符串的函数：MAKE_SET(x,s1,s2,...)

MAKE_SET(x,s1,s2,...): 返回由x的二进制数指定的相应位的字符串组成的字符串，s1对应比特1,s2对应比特01,依此类推。s1,s2...中的NULL值不会被添加到结果中。

mysql> select MAKE_SET(1,'a','b','c') as c1, MAKE_SET(1|4,'hello','nice','world') as c2, MAKE_SET(1|4,'hello','nice',NULL,'world') as c3, MAKE_SET(0,'a','b','c') as c4;

+----+-------------+-------+----+

| c1 | c2 | c3 | c4 |

+----+-------------+-------+----+

| a | hello,world | hello | |

+----+-------------+-------+----+

说明：

1的二进制值为0001, 4的二进制值为0100，1和4进行或操作之后的二进制值为0101，从右到左第1位和第3位为１。

MAKE_SET(1,’a’,’b’,’c’): 返回第1个字符串；

MAKE_SET(1|4,'hello','nice','world')：返回从左端开始第1和第3个字符组成的字符串；

MAKE_SET(1|4,'hello','nice',NULL,'world')：NULL值不会添加到结果中，因此只会返回第一个字符串；

MAKE_SET(0,'a','b','c'): 返回空字符串。

如果您们在尝试的过程中遇到什么问题或者我的代码有错误的地方，请给予指正，非常感谢！

联系方式：david.louis.tian@outlook.com

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