MySQL函数学习笔记二：字符函数_MySQL
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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