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一、数学函数

1、ABS(x)

返回绝对值。

mysql> select abs(-4);
+---------+
| abs(-4) |
+---------+
|       4 |
+---------+
1 row in set (0.00 sec)

2、PI（）

返回圆周率，并四舍五入保留五位小数。

mysql> select pi();
+----------+
| pi()     |
+----------+
| 3.141593 |
+----------+
1 row in set (0.00 sec)

3、RAND()和RAND(X)

返回一个0-1的随机浮点数，包含0和1，如果指定值则它被用作种子值，用来产生重复序列。

mysql> select rand(),rand(2),rand(2);
+-------------------+--------------------+--------------------+
| rand()            | rand(2)            | rand(2)            |
+-------------------+--------------------+--------------------+
| 0.999596054665368 | 0.6555866465490187 | 0.6555866465490187 |
+-------------------+--------------------+--------------------+
1 row in set (0.00 sec)

4、SQRT(X)

平方根函数，返回一个非负数的平方根，如果x为负数则返回NULL值。

mysql> select sqrt(2),sqrt(9),sqrt(-25);
+--------------------+---------+-----------+
| sqrt(2)            | sqrt(9) | sqrt(-25) |
+--------------------+---------+-----------+
| 1.4142135623730951 |       3 |      NULL |
+--------------------+---------+-----------+
1 row in set (0.00 sec)

5、MOD(X,Y)

求余数函数，也叫取模函数

mysql> select mod(7,2),mod(9,3);
+----------+----------+
| mod(7,2) | mod(9,3) |
+----------+----------+
|        1 |        0 |
+----------+----------+
1 row in set (0.00 sec)

6、CEIL(X)、CEILING(X)

取整函数，向上取整，取X整数部分再加1。

mysql> select ceil(3.78),ceil(7.43),ceiling(4.78),ceiling(2.23);
+------------+------------+---------------+---------------+
| ceil(3.78) | ceil(7.43) | ceiling(4.78) | ceiling(2.23) |
+------------+------------+---------------+---------------+
|          4 |          8 |             5 |             3 |
+------------+------------+---------------+---------------+
1 row in set (0.00 sec)

7、FLOOR(X)

取整函数，向下取整，只取X整数部分。

mysql> select floor(6.98),floor(3.25);
+-------------+-------------+
| floor(6.98) | floor(3.25) |
+-------------+-------------+
|           6 |           3 |
+-------------+-------------+
1 row in set (0.00 sec)

8、ROUND(X)、ROUND（X,Y）

ROUND(X)取整函数，取X接近的整数，对X的值进行四舍五入；ROUND（X,Y）返回X参数，指定Y个小数位，也是对小数部分Y后面的位数进行四舍五入。

mysql> select round(8.76),round(56.765,2);
+-------------+-----------------+
| round(8.76) | round(56.765,2) |
+-------------+-----------------+
|           9 |           56.77 |
+-------------+-----------------+
1 row in set (0.00 sec)

9、POW(X,Y)、POWER(X,Y)、EXP(X)

POW(X,Y)、POWER(X,Y)返回的结果一样，X的Y次方，EXP(X)的是e的X次方，e的近似值为2.71828 。

mysql> select pow(2,3),power(2,10),exp(2);
+----------+-------------+------------------+
| pow(2,3) | power(2,10) | exp(2)           |
+----------+-------------+------------------+
|        8 |        1024 | 7.38905609893065 |
+----------+-------------+------------------+
1 row in set (0.01 sec)

10、LOG(X)、LOGN(X)

LOG(X)返回x的自然对数e，LOGN(X)是以N为底数的X的对数。

mysql> select log(2),log2(2),log10(100);
+--------------------+---------+------------+
| log(2)             | log2(2) | log10(100) |
+--------------------+---------+------------+
| 0.6931471805599453 |       1 |          2 |
+--------------------+---------+------------+
1 row in set (0.00 sec)

二、字符串函数

1、CHAR_LENGTH(STR)、LENGTH(STR)

CHAR_LENGTH(STR)返回值为字符串STR所有包含的字符个数，一个多字符算作一个单字符，LENGTH(STR)返回值为字符串的字节长度，使用UTF8的一种可变长字符编码，一个汉字是三个字节，一个数字或者一个字母算一个字节。

mysql> select char_length('aabb'),char_length('lllsss'),length('哈哈哈'),length('shsj211');
+---------------------+-----------------------+---------------------+-------------------+
| char_length('aabb') | char_length('lllsss') | length('哈哈哈')    | length('shsj211') |
+---------------------+-----------------------+---------------------+-------------------+
|                   4 |                     6 |                   9 |                 7 |
+---------------------+-----------------------+---------------------+-------------------+
1 row in set (0.00 sec)

2、CONCAT(S1,S2,……)

返回的值结果为连接参数产生的字符串，可一个或多个参数，如过其中一个参数为NULL则返回值就为NULL。

mysql> select concat('hhsh','988','   ll'),concat('hdjh',null,'ooio');
+------------------------------+----------------------------+
| concat('hhsh','988','   ll') | concat('hdjh',null,'ooio') |
+------------------------------+----------------------------+
| hhsh988   ll                 | NULL                       |
+------------------------------+----------------------------+
1 row in set (0.00 sec)

3、CONCAT_WS（X,S1,S2……）

x参数是分隔符，分隔符在两个连续的字符串之间。分隔符可以是一个字串，也可以是其他字符，如果分隔符为NULL则结果返回为NULL，函数会忽略任何分隔符参数后的NULL值，其他参数为NULL值，不会影响拼接。

mysql> select concat_ws('=>','hdh','kkk','lll','yyy'),concat_ws('-','iii',null,'ppp'),concat_ws(null,'kkk','ppp','uuu');         
+-----------------------------------------+---------------------------------+-----------------------------------+
| concat_ws('=>','hdh','kkk','lll','yyy') | concat_ws('-','iii',null,'ppp') | concat_ws(null,'kkk','ppp','uuu') |
+-----------------------------------------+---------------------------------+-----------------------------------+
| hdh=>kkk=>lll=>yyy                      | iii-ppp                         | NULL                              |
+-----------------------------------------+---------------------------------+-----------------------------------+
1 row in set (0.00 sec)

4、INSERT（s1,x,len,s2）

替换字符串函数，返回字符串s1，x为替换替换字符的起始位置，len为替换的长度，s2为替换的字符或字符串。

mysql> select insert('aaahhhss',4,3,'iii');
+------------------------------+
| insert('aaahhhss',4,3,'iii') |
+------------------------------+
| aaaiiiss                     |
+------------------------------+
1 row in set (0.00 sec)

5、大小写字母转换函数

LOWER(STR)或LCASE(STR)可以将字符串STR字符替换为小写字母。UPPER(STR)可以将字符串STR字符串字符替换为大写字母。

mysql> select lower('AAA'),LCASE('BBHHH'),UPPER('kkkk');
+--------------+----------------+---------------+
| lower('AAA') | LCASE('BBHHH') | UPPER('kkkk') |
+--------------+----------------+---------------+
| aaa          | bbhhh          | KKKK          |
+--------------+----------------+---------------+
1 row in set (0.00 sec)

 6、LEFT(STR,N)、RIGHT(STR,N)

LEFT（STR，N）返回字符串STR开始的最左边N个字符串，RIGHT（STR,N）返回字符串STR最右边N个字符串。

mysql> select left('hhshkkss',4),right('uuuaaa',3);
+--------------------+-------------------+
| left('hhshkkss',4) | right('uuuaaa',3) |
+--------------------+-------------------+
| hhsh               | aaa               |
+--------------------+-------------------+
1 row in set (0.00 sec)

7、LTRIM(STR)、RTRIM(STR)、TRIM(STR)

LTRIM(STR)返回去掉左边空格的字符串、RTRIM(STR)返回去掉右边空格的字符串、TRIM(STR)去掉两边空格后的字符串。

mysql> select ltrim('   jjhjhj  '),rtrim('   djfijd     '),trim('    jhjdhfj  ');
+----------------------+-------------------------+-----------------------+
| ltrim('   jjhjhj  ') | rtrim('   djfijd     ') | trim('    jhjdhfj  ') |
+----------------------+-------------------------+-----------------------+
| jjhjhj               |    djfijd               | jhjdhfj               |
+----------------------+-------------------------+-----------------------+
1 row in set (0.00 sec)

8、SPACE(N)

产生N个空格字符。

mysql> select concat('[',space(5),']');
+--------------------------+
| concat('[',space(5),']') |
+--------------------------+
| [     ]                  |
+--------------------------+
1 row in set (0.00 sec)

9、REPLACE（S,STR1,STR2）

返回使用字符串STR2替代字符串S中所有的字符串STR1。

mysql> select replace('aabb','a','c');
+-------------------------+
| replace('aabb','a','c') |
+-------------------------+
| ccbb                    |
+-------------------------+
1 row in set (0.00 sec)

10、STRCMP（STR1,STR2）

比较字符串函数，若STR1等于STR2就返回0，若STR1大于STR2就返回1，若字符串STR1小于STR2就返回-1。

mysql> select strcmp('aa','aa'),strcmp('aa1','aa'),strcmp('aa','aa1');
+-------------------+--------------------+--------------------+
| strcmp('aa','aa') | strcmp('aa1','aa') | strcmp('aa','aa1') |
+-------------------+--------------------+--------------------+
|                 0 |                  1 |                 -1 |
+-------------------+--------------------+--------------------+
1 row in set (0.00 sec)

11、REVERSE(STR)

逆置字符串函数。

mysql> select reverse('hhhsssbbb');
+----------------------+
| reverse('hhhsssbbb') |
+----------------------+
| bbbssshhh            |
+----------------------+
1 row in set (0.00 sec)

12、LOCATE(STR1,STR), POSITION(STR1 IN STR)

返回STR1在字符串STR中的起始位置

mysql> select locate('ss','aaaddsskkll'),position('bb' in 'shdhjavbbjkj');
+----------------------------+----------------------------------+
| locate('ss','aaaddsskkll') | position('bb' in 'shdhjavbbjkj') |
+----------------------------+----------------------------------+
|                          6 |                                8 |
+----------------------------+----------------------------------+
1 row in set (0.00 sec)

三、日期函数

1、CURDATE()、CURRENT_DATE()

获取当前日期，格式为YYYY-MM-DD格式

mysql> select curdate(),current_date();
+------------+----------------+
| curdate()  | current_date() |
+------------+----------------+
| 2023-08-19 | 2023-08-19     |
+------------+----------------+
1 row in set (0.00 sec)

2、CURRENT_TIMESTAMP()、LOCATIME()、NOW()、SYSDATE()

获取当前日期和时间

mysql> select current_timestamp(),localtime(),now(),sysdate();
+---------------------+---------------------+---------------------+---------------------+
| current_timestamp() | localtime()         | now()               | sysdate()           |
+---------------------+---------------------+---------------------+---------------------+
| 2023-08-19 19:17:53 | 2023-08-19 19:17:53 | 2023-08-19 19:17:53 | 2023-08-19 19:17:53 |
+---------------------+---------------------+---------------------+---------------------+
1 row in set (0.00 sec)

3、YEAR(DATE)

获取日期月份

mysql> select year(now());
+-------------+
| year(now()) |
+-------------+
|        2023 |
+-------------+
1 row in set (0.00 sec)

4、MONTH(DATE)、MONTHNAME(DATE)

获取日期月份，MONTHNAME(DATE)格式为英文。

mysql> select month(now()),monthname(now());
+--------------+------------------+
| month(now()) | monthname(now()) |
+--------------+------------------+
|            8 | August           |
+--------------+------------------+
1 row in set (0.00 sec)

5、DAY(DATE)

获取具体的日

mysql> select day(now());
+------------+
| day(now()) |
+------------+
|         19 |
+------------+
1 row in set (0.00 sec)

6、DAYNME(DTAE)、DAYOFWEEK(DATE)、WEEKDAY(DATE)

获取星期几

mysql> select dayname(now()),dayofweek(now()),weekday(now());
+----------------+------------------+----------------+
| dayname(now()) | dayofweek(now()) | weekday(now()) |
+----------------+------------------+----------------+
| Saturday       |                7 |              5 |
+----------------+------------------+----------------+
1 row in set (0.00 sec)

7、DAYOFYEAR(DATE)、DAYOFMONTH(DATE)

DAYOFYEAR(DATE)返回值为这一年的第几天。

DAYOFMONTH(DATE)返回值为这一年的第几月。

mysql> select dayofmonth(now()),dayofyear(now());
+-------------------+------------------+
| dayofmonth(now()) | dayofyear(now()) |
+-------------------+------------------+
|                19 |              231 |
+-------------------+------------------+
1 row in set (0.00 sec)

8、TIME_TO_SEC(time)

将时间转换为秒。

mysql> select time_to_sec('14:20:30'),time_to_sec(time(now()));
+-------------------------+--------------------------+
| time_to_sec('14:20:30') | time_to_sec(time(now())) |
+-------------------------+--------------------------+
|                   51630 |                    70443 |
+-------------------------+--------------------------+
1 row in set (0.00 sec)

四、条件判断函数

1、IF(EXPR,V1,V2)

如果表达式EXPR为TRUE就返回V1，否则返回V2，IF()函数返回的值可以是数字值或者字符串值。

mysql> select if(2>6,'YES','NO');
+--------------------+
| if(2>6,'YES','NO') |
+--------------------+
| NO                 |
+--------------------+
1 row in set (0.00 sec)

2、IFNULL(V1,V2)

返回值如果V1不会NULL就返回V1，否则返回V2。

mysql> select ifnull(null,'aa'),ifnull(123,777);
+-------------------+-----------------+
| ifnull(null,'aa') | ifnull(123,777) |
+-------------------+-----------------+
| aa                |             123 |
+-------------------+-----------------+
1 row in set (0.00 sec)

3、CASE EXPR WHEN V1 THEN R1 ……[ELSE Rn]  END

返回值为当EXPR的值等于Vn就返回对应Rn的值。当都不满足Vn的值有ELSE语句就返回后的值。

mysql> select case 3-> when 1 then 'a'-> when 2 then 'b'-> when 3 then 'c'-> when 4 then 'd'-> else 'e' end;
+-------------------------------------------------------------------------------------+
| case 3
when 1 then 'a'
when 2 then 'b'
when 3 then 'c'
when 4 then 'd'
else 'e' end |
+-------------------------------------------------------------------------------------+
| c                                                                                   |
+-------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

五、系统信息函数

1、VERSION()、CONNECTION_ID()、DATABASE()

返回MySQL的版本号、连接数、数据库名的函数。

mysql> select version(),connection_id(),database();
+-----------+-----------------+------------+
| version() | connection_id() | database() |
+-----------+-----------------+------------+
| 5.7.18    |               3 | db1        |
+-----------+-----------------+------------+
1 row in set (0.00 sec)

2、USER()、SESSION_USER()

返回当前登录用户名，连接会话的用户。

mysql> select USER(),SESSION_USER();
+----------------+----------------+
| USER()         | SESSION_USER() |
+----------------+----------------+
| root@localhost | root@localhost |
+----------------+----------------+
1 row in set (0.00 sec)

六、加/解密函数

1、PASSWORD('password')

返回一个加密后的字符串，此函数是单向加密不可逆。

mysql> select password('123aaabbb');
+-------------------------------------------+
| password('123aaabbb')                     |
+-------------------------------------------+
| *616E8E68832C222FFAEB396A9FA75865A6F7F039 |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)

2、MD5('password')

返回一个MD5 128比特校验和，该值以32位十六进制数字的二进制字符串形式返回。若参数为NULL则返回NULL。

mysql> select md5('hhhss12234'),md5(null);
+----------------------------------+-----------+
| md5('hhhss12234')                | md5(null) |
+----------------------------------+-----------+
| 2c6efa51fe416f1c23d716bab5be9e4e | NULL      |
+----------------------------------+-----------+
1 row in set (0.00 sec)

3、ENCODE(STR，PSWD_STR)

使用PSWD作为密码，加密STR，使用DECODE（）解密结果，是一个和STR长度相同的二进制字符串。

mysql> select encode('123hhh','aaa123');
+---------------------------+
| encode('123hhh','aaa123') |
+---------------------------+
| sbx'                     |
+---------------------------+
1 row in set, 1 warning (0.00 sec)

4、DECODE(CRYPT_STR,PASSD_STR)

使用PASSD_STR作为密码，解密加密字符串CRYPT_STR，CRYPT_STR是由ENCODE（）返回的字符串。

mysql> select decode(encode('123hhh','aaa123'),'aaa123');
+--------------------------------------------+
| decode(encode('123hhh','aaa123'),'aaa123') |
+--------------------------------------------+
| 123hhh                                     |
+--------------------------------------------+
1 row in set, 2 warnings (0.00 sec)

参考书籍：MySQL5.7从入门到精通-清华大学出版社