mysql5.5从零开始_【MySQL函数】MySQL 5.5从零开始学第六章

说明：本文总结自：《MySQL 5.5从零开始学》第六章

MySQL中的函数包括：

数学函数、字符串函数、日期和时间函数、条件判断函数、系统信息函数和加密函数等。

函数：

表示对输入参数值返回一个具有特定关系的值。

一、数学函数

主要的数学函数有：绝对值函数、三角函数(包括正弦函数、余弦函数、正切函数、余切函数等)、

对数函数、随机函数等。(PS:在有错误产生时，数学函数将会返回空值NULL)

1.1 绝对值函数ABS(x)

mysql> SELECT ABS(1),ABS(-1.1),ABS(-33);

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

| ABS(1) | ABS(-1.1) | ABS(-33) |

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

| 1 | 1.1 | 33 |

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

1 row in set (0.03 sec)

1.2返回圆周率函数PI()

mysql> SELECT PI();

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

| PI() |

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

| 3.141593 |

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

1 row in set (0.00 sec)

1.3平方根函数SQRT(x)

PS:SQRT(x)范围非负数x的二次方根

mysql> SELECT SQRT(9),SQRT(10),SQRT(-9);

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

| SQRT(9) | SQRT(10) | SQRT(-9) |

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

| 3 | 3.1622776601683795 | NULL |--负数没有平方根

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

1 row in set (0.00 sec)

1.4求余函数MOD(x,y)

mysql> select MOD(5,2),MOD(7,4),MOD(50,8);

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

| MOD(5,2) | MOD(7,4) | MOD(50,8) |

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

| 1 | 3 | 2 |

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

1 row in set (0.00 sec)

1.5获取整数的函数CEIL(x)、CEILING(x)、FLOOR(x)

PS:CEIL(x)、CEILING(x)意义相同

mysql> SELECT CEIL(-2.26),CEIL(2.26),CEILING(-2.26),CEILING(2.26);

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

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

| -2 | 3 | -2 | 3 |

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

1 row in set (0.00 sec)

mysql> SELECT FLOOR(-2.26),FLOOR(2.26);

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

| FLOOR(-2.26) | FLOOR(2.26) |

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

| -3 | 2 |

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

1 row in set (0.00 sec)

1.6获取随机数的函数RAND()和RAND(x)

PS:a.RAND(x)返回一个随机浮点值v，范围在0到1之间。若已制定一个整数参数x，则它被用作种子值，用来产生重复序列。

b.不带参数的RAND()每次产生的随机数值不同

c.当RAND()参数相同时，将产生相同的随机数，不同的x产生的随机数值不同

mysql> SELECT RAND(),RAND(),RAND();

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

| RAND() | RAND() | RAND() |

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

| 0.0917864060884215 | 0.6539400337766298 | 0.9943409813515293 |

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

1 row in set (0.00 sec)

mysql> SELECT RAND(5),RAND(5),RAND(6);

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

| RAND(5) | RAND(5) | RAND(6) |

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

| 0.40613597483014313 | 0.40613597483014313 | 0.6563190842571847 |

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

1 row in set (0.00 sec)

1.7四舍五入函数ROUND(x)、ROUND(x,y)和TRUNCATE(x,y)

PS:a.ROUND(x,y)当y值为负数时，保留的小数点左边的相应位数直接保存为0，不进行四舍五入

b.ROUND(x,y)函数在截取值的时候会四舍五入，而TRUNCATE(x,y)直接截取值，不进行四舍五入

mysql> SELECT ROUND(-2.34),ROUND(-2.56),ROUND(2.34),ROUND(2.56);

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

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

| -2 | -3 | 2 | 3 |

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

1 row in set (0.00 sec)

mysql> SELECT ROUND(-2.34,1),ROUND(-20.56,-1),ROUND(2.35,1),ROUND(25.56,-1);

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

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

| -2.3 | -20 | 2.4 | 30 |

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

1 row in set (0.00 sec)

mysql> SELECT TRUNCATE(-2.34,1),TRUNCATE(-20.56,-1),TRUNCATE(2.35,1),TRUNCATE(25.56,-1);

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

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

| -2.3 | -20 | 2.3 | 20 |

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

1 row in set (0.00 sec)

1.8符号函数SIGN(x)

PS:SIGN(x)返回参数的符号，x的值为负、零或正时结果依次为-1、0或1.

mysql> SELECT SIGN(-33),SIGN(0),SIGN(33);

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

| SIGN(-33) | SIGN(0) | SIGN(33) |

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

| -1 | 0 | 1 |

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

1 row in set (0.00 sec)

1.9幂运算函数POW(x,y)、POWER(x,y)和EXP(x)

PS:a.POW(x,y)、POWER(x,y)意义相同

b.EXP(x)返回e的x乘方后的值(e的值是多少？答：e=2.71828？？？？)

mysql> SELECT POW(2,2),POW(2,-2),POWER(2,2),POW(2,-2);

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

| POW(2,2) | POW(2,-2) | POWER(2,2) | POW(2,-2) |

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

| 4 | 0.25 | 4 | 0.25 |

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

1 row in set (0.00 sec)

mysql> SELECT EXP(3),EXP(-3),EXP(0);

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

| EXP(3) | EXP(-3) | EXP(0) |

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

| 20.085536923187668 | 0.049787068367863944 | 1 |

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

1 row in set (0.00 sec)

1.10对数运算函数LOG(x)和LOG10(x)

PS:a.LOG(x)返回x的自然对数，x相对于基数e的对数。(什么是对数？答：如果a的x次方等于N(a>0，且a不等于1)，那么数x叫做以a为底N的对数(logarithm)，记作x=logaN。其中，a叫做对数的底数，N叫做真数。)

b.对数定义域不能为负数，因此LOG(-5)返回结果为NULL

mysql> SELECT LOG(5),LOG(-5);

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

| LOG(5) | LOG(-5) |

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

| 1.6094379124341003 | NULL |

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

1 row in set (0.00 sec)

mysql> SELECT LOG10(2),LOG10(100),LOG10(-100);

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

| LOG10(2) | LOG10(100) | LOG10(-100) |

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

| 0.3010299956639812 | 2 | NULL |

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

1 row in set (0.00 sec)

1.11角度与弧度相互转换的函数RADIANS(x)和DEGREES(x)

mysql> SELECT RADIANS(90),RADIANS(180);

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

| RADIANS(90) | RADIANS(180) |

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

| 1.5707963267948966 | 3.141592653589793 |

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

1 row in set (0.00 sec)

mysql> SELECT DEGREES(PI()),DEGREES(PI()/2);

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

| DEGREES(PI()) | DEGREES(PI()/2) |

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

| 180 | 90 |

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

1 row in set (0.00 sec)

1.12正弦函数SIN(x)和反正弦函数ASIN(x)

PS:a.余弦函数COS(x),反余弦函数ACOS(x),正切函数TAN(x),反正切函数ATAN(x),余切函数COT(x)情况类似

b.ASIN(x)返回x的反正弦，若x不在-1到1的范围之内，则返回NULL。

mysql> SELECT SIN(1),ROUND(SIN(PI()));

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

| SIN(1) | ROUND(SIN(PI())) |

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

| 0.8414709848078965 | 0 |

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

1 row in set (2.68 sec)

mysql> SELECT ASIN(0.8414709848078965),ASIN(3);

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

| ASIN(0.8414709848078965) | ASIN(3) |

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

| 1 | NULL |

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

1 row in set (0.01 sec)

二、字符串函数

2.1计算字符串字符数的函数CHAR_LENGTH(str)和字符串长度的函数LENGTH(str)

mysql> SELECT CHAR_LENGTH('zhangmeng'),CHAR_LENGTH(1234);

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

| CHAR_LENGTH('zhangmeng') | CHAR_LENGTH(1234) |

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

| 9 | 4 |

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

1 row in set (0.01 sec)

mysql> SELECT LENGTH('zhangmeng'),LENGTH(1234);

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

| LENGTH('zhangmeng') | LENGTH(1234) |

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

| 9 | 4 |

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

1 row in set (0.00 sec)

2.2合并字符串函数CONCAT(s1,s2,...)、CONCAT_WS(x,s1,s2,...)

PS:a.CONCAT(s1,s2,...)如何有任何一个参数为NULL，则返回值为NULL。

b.CONCAT_WS(x,s1,s2,...)，使用分隔符x将两个字符串连接成一个字符串，同时忽略NULL值

mysql> SELECT CONCAT('MySQL',5.6),CONCAT('MySQL',NULL,5.6);

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

| CONCAT('MySQL',5.6) | CONCAT('MySQL',NULL,5.6) |

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

| MySQL5.6 | NULL |

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

1 row in set (0.00 sec)

mysql> SELECT CONCAT_WS('-','MySQL',5.6),CONCAT_WS('*','MySQL',NULL,5.6);

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

| CONCAT_WS('-','MySQL',5.6) | CONCAT_WS('*','MySQL',NULL,5.6) |

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

| MySQL-5.6 | MySQL*5.6 |

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

1 row in set (0.00 sec)

2.3替换字符串的函数INSERT(s1,x,len,s2)

mysql> SELECT INSERT('15271937914',4,4,'****');

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

| INSERT('15271937914',4,4,'****') |

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

| 152****7914 |

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

1 row in set (0.00 sec)

2.4字母大写转小写函数LOWER(str)、LCASE(str)

PS:LOWER(str)、LCASE(str)意义相同

mysql> SELECT LOWER('SELECT'),LCASE('DELETE');

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

| LOWER('SELECT') | LCASE('DELETE') |

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

| select | delete |

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

1 row in set (0.00 sec)

2.5字母小写转大写函数UPPER(str)、UCASE(str)

PS:UPPER(str)、UCASE(str)意义相同

mysql> SELECT UPPER('select'),UCASE('Delete');

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

| UPPER('select') | UCASE('delete') |

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

| SELECT | DELETE |

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

1 row in set (0.00 sec)

2.6获取制定长度的字符串的函数LEFT(s,n)和RIGHT(s,n)

mysql> SELECT LEFT('zhangmeng',5);

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

| LEFT('zhangmeng',5) |

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

| zhang |

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

1 row in set (0.00 sec)

mysql> SELECT RIGHT('zhangmeng',4);

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

| RIGHT('zhangmeng',4) |

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

| meng |

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

1 row in set (0.00 sec)

2.7填充字符串函数LPAD(s1,len,s2)和RPAD(s1,len,s2)

mysql> SELECT LPAD('hello',4,'*'),LPAD('hello',9,'*');

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

| LPAD('hello',4,'*') | LPAD('hello',9,'*') |

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

| hell | ****hello |

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

1 row in set (0.00 sec)

mysql> SELECT RPAD('hello',4,'*'),RPAD('hello',9,'*');

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

| RPAD('hello',4,'*') | RPAD('hello',9,'*') |

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

| hell | hello**** |

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

1 row in set (0.00 sec)

2.8删除空格的函数LTRIM(s)、RTRIM(s)、TRIM(s)

mysql> SELECT '( ball )',CONCAT('(',LTRIM(' ball '),')');

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

| ( ball ) | CONCAT('(',LTRIM(' ball '),')') |

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

| ( ball ) | (ball ) |

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

1 row in set (0.00 sec)

mysql> SELECT '( ball )',CONCAT('(',RTRIM(' ball '),')');

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

| ( ball ) | CONCAT('(',RTRIM(' ball '),')') |

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

| ( ball ) | ( ball) |

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

1 row in set (0.00 sec)

mysql> SELECT '( ball )',CONCAT('(',TRIM(' ball '),')');

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

| ( ball ) | CONCAT('(',TRIM(' ball '),')') |

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

| ( ball ) | (ball) |

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

1 row in set (0.00 sec)

2.9删除指定字符串的函数TRIM(s1 FROM s)

mysql> SELECT TRIM('he' FROM 'hello');

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

| TRIM('he' FROM 'hello') |

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

| llo |

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

1 row in set (0.00 sec)

2.10重复生成字符串的函数REPEAT(s,n)

mysql> SELECT REPEAT('hello',2);

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

| REPEAT('hello',2) |

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

| hellohello |

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

1 row in set (0.00 sec)

2.11空格函数SPACE(n)

mysql> SELECT CONCAT('(',SPACE(8),')');

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

| CONCAT('(',SPACE(8),')') |

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

| ( ) |

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

1 row in set (0.00 sec)

2.12替换函数REPLACE(s,s1,s2)

mysql> SELECT REPLACE('xxx.mysql.com','x','w');

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

| REPLACE('xxx.mysql.com','x','w') |

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

| www.mysql.com |

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

1 row in set (0.00 sec)

2.13比较字符串的大小函数STRCMP(s1,s2)

mysql> SELECT STRCMP('test','test'),STRCMP('test','test1'),STRCMP('test1','test');

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

| STRCMP('test','test') | STRCMP('test','test1') | STRCMP('test1','test') |

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

| 0 | -1 | 1 |

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

1 row in set (0.00 sec)

2.14获取子串的函数SUBSTRING(s,n,len)和MID(s,n,len)

PS:SUBSTRING(s,n,len)和MID(s,n,len)意义相同

mysql> SELECT SUBSTRING('breakfast',5) AS col1,SUBSTRING('breakfast',5,3) AS col2,SUBSTRING('lunch',-3),SUBSTRING('lunch',-5,3);

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

| col1 | col2 | SUBSTRING('lunch',-3) | SUBSTRING('lunch',-5,3) |

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

| kfast | kfa | nch | lun |

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

1 row in set (0.00 sec)

2.15匹配子串开始位置的函数LOCATE(str1,str)、POSITION(str1 IN str)、INSTR(str,str1)

mysql> SELECT LOCATE('ball','football'),POSITION('ball'in'football'),INSTR('football','ball');

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

| LOCATE('ball','football') | POSITION('ball'in'football') | INSTR('football','ball') |

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

| 5 | 5 | 5 |

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

1 row in set (0.00 sec)

2.16字符串逆序的函数REVERSE(s)

mysql> SELECT REVERSE('12345');

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

| REVERSE('12345') |

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

| 54321 |

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

1 row in set (0.00 sec)

2.17返回制定位置的字符串的函数ELT(n,str1,str2,...)

mysql> SELECT ELT(2,'1st','2st','3st'),ELT(4,'1st','2st','3st');

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

| ELT(2,'1st','2st','3st') | ELT(4,'1st','2st','3st') |

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

| 2st | NULL |

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

1 row in set (0.00 sec)

2.18返回制定字符串位置的函数FIELD(s,s1,s2,..)和FIND_IN_SET(s1,s2)

PS:FIELD(s,s1,s2,..)和FIND_IN_SET(s1,s2)意义相同

mysql> SELECT FIELD('hi','hello','hi','hey');

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

| FIELD('hi','hello','hi','hey') |

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

| 2 |

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

1 row in set (0.00 sec)

mysql> SELECT FIND_IN_SET('hi','hello,hi,hey');

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

| FIND_IN_SET('hi','hello,hi,hey') |

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

| 2 |

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

1 row in set (0.00 sec)

三、日期和时间函数

3.1获取当前日期的函数CURDATE()、CURRENT_DATE()

PS:a.CURDATE()、CURRENT_DATE()意义相同

b.CURDATE()+0将当前日期值转换为数值型

mysql> SELECT CURDATE(),CURRENT_DATE(),CURDATE()+0;

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

| CURDATE() | CURRENT_DATE() | CURDATE()+0 |

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

| 2017-07-13 | 2017-07-13 | 20170713 |

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

1 row in set (0.02 sec)

3.2获取当前时间和函数CURTIME()和CURRENT_TIME()

PS:CURTIME()和CURRENT_TIME()意义相同

mysql> SELECT CURTIME(),CURRENT_TIME(),CURTIME()+0;

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

| CURTIME() | CURRENT_TIME() | CURTIME()+0 |

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

| 17:45:39 | 17:45:39 | 174539 |

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

1 row in set (0.00 sec)

3.3获取当前日期和时间的函数CURRENT_TIMESTAMP(),LOCALTIME(),NOW(),SYSDATE()

PS:CURRENT_TIMESTAMP(),LOCALTIME(),NOW(),SYSDATE()意义相同

mysql> SELECT CURRENT_TIMESTAMP(),LOCALTIME(),NOW(),SYSDATE();

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

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

| 2017-07-13 17:49:54 | 2017-07-13 17:49:54 | 2017-07-13 17:49:54 | 2017-07-13 17:49:54 |

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

1 row in set (0.00 sec)

3.4返回UTC日期的函数UTC_DATE()和返回UTC时间的函数UTC_TIME()

PS:UTC:世界标准时间

mysql> SELECT UTC_DATE(),UTC_TIME(),UTC_DATE()+0;

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

| UTC_DATE() | UTC_TIME() | UTC_DATE()+0 |

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

| 2017-07-13 | 09:58:22 | 20170713 |

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

1 row in set (0.00 sec)

3.5获取月份的函数MONTH(date)和MONTHNAME(date)

mysql> SELECT MONTH('2017-07-13'),MONTHNAME('2017-07-13');

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

| MONTH('2017-07-13') | MONTHNAME('2017-07-13') |

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

| 7 | July |

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

1 row in set (0.00 sec)

3.6获取星期的函数DAYNAME(d)、DAYOFWEEK(d)和WEEKDAY(d)

PS:DAYOFWEEK(d)和WEEKDAY(d)函数都是返回指定日期在某一周内的位置

mysql> SELECT DAYNAME('2017-07-13'),DAYOFWEEK('2017-07-13'),WEEKDAY('2017-07-13');

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

| DAYNAME('2017-07-13') | DAYOFWEEK('2017-07-13') | WEEKDAY('2017-07-13') |

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

| Thursday | 5 | 3 |

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

3.7获取星期数的函数WEEK(d)

PS:WEEK(d)函数查询指定日期是一年中的第几周

mysql> SELECT WEEK('2017-07-13');

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

| WEEK('2017-07-13') |

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

| 28 |

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

1 row in set (0.00 sec)

3.8获取天数的函数DAYOFYEAR(d)

PS:DAYOFYEAR(d)函数查询指定日期是一年中的第几天

mysql> SELECT DAYOFYEAR('2017-07-13');

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

| DAYOFYEAR('2017-07-13') |

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

| 194 |

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

1 row in set (0.00 sec)

3.9获取年份的函数year(d)

mysql> SELECT YEAR('2017-07-13');

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

| YEAR('2017-07-13') |

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

| 2017 |

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

1 row in set (0.00 sec)

3.10计算日期的函数(加)DATE_ADD(date,interval expr type),ADDDATE(date,interval expr type)

mysql> SELECT DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 HOUR),

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 DAY),

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 MONTH),

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 YEAR) \G;

*************************** 1. row ***************************

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 HOUR): 2017-07-13 20:04:30

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 DAY): 2017-07-14 19:04:30

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 MONTH): 2017-08-13 19:04:30

DATE_ADD('2017-07-13 19:04:30',INTERVAL 1 YEAR): 2018-07-13 19:04:30

1 row in set (0.00 sec)

mysql> SELECT ADDDATE('2017-07-13 19:04:30',INTERVAL 1 HOUR),

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 DAY),

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 MONTH),

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 YEAR) \G;

*************************** 1. row ***************************

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 HOUR): 2017-07-13 20:04:30

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 DAY): 2017-07-14 19:04:30

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 MONTH): 2017-08-13 19:04:30

ADDDATE('2017-07-13 19:04:30',INTERVAL 1 YEAR): 2018-07-13 19:04:30

1 row in set (0.01 sec)

3.11计算日期的函数(减)DATE_SUB(date,interval expr type),SUBDATE(date,interval expr type)

mysql> SELECT DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 HOUR),

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 DAY),

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 MONTH),

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 YEAR) \G;

*************************** 1. row ***************************

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 HOUR): 2017-07-13 18:04:30

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 DAY): 2017-07-12 19:04:30

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 MONTH): 2017-06-13 19:04:30

DATE_SUB('2017-07-13 19:04:30',INTERVAL 1 YEAR): 2016-07-13 19:04:30

1 row in set (0.00 sec)

mysql> SELECT SUBDATE('2017-07-13 19:04:30',INTERVAL 1 HOUR),

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 DAY),

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 MONTH),

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 YEAR) \G;

*************************** 1. row ***************************

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 HOUR): 2017-07-13 18:04:30

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 DAY): 2017-07-12 19:04:30

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 MONTH): 2017-06-13 19:04:30

SUBDATE('2017-07-13 19:04:30',INTERVAL 1 YEAR): 2016-07-13 19:04:30

1 row in set (0.00 sec)

3.12计算两个日期之间的间隔天数DATEDIFF(date1,date2)

PS:DATEDIFF(date1,date2)函数返回起始时间date1和结束时间date2之间的天数。date1和date2为日期或

date-and-time表达式，计算中只用到这些值的日期部分。

mysql> SELECT DATEDIFF('2017-07-16 12:51:30','2017-07-13 18:30:30');

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

| DATEDIFF('2017-07-16 12:51:30','2017-07-13 18:30:30') |

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

| 3 |

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

1 row in set (0.00 sec)

3.13格式化输出日期和时间的函数DATE_FORMAT()

mysql> SELECT DATE_FORMAT('2017-07-16 12:39:30','%W %M %p %Y' );

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

| DATE_FORMAT('2017-07-16 12:39:30','%W %M %p %Y' ) |

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

| Sunday July PM 2017 |

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

1 row in set (0.00 sec)

四、条件判断函数

4.1条件判断函数IF(expr,v1,v2)

PS:IF(expr,v1,v2)如果表达式expr是TRUE(expr<>0 and expr<>NULL),则IF()的返回值为v1，否则返回值v2.

mysql> SELECT IF(1>2,3,4),IF(1<2,'yes','no'),IF(STRCMP('test','test1'),'no','yes');

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

| IF(1>2,3,4) | IF(1<2,'yes','no') | IF(STRCMP('test','test1'),'no','yes') |

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

| 4 | yes | no |

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

1 row in set (0.00 sec)

4.2条件判断函数IFNULL()

PS:a.IFNULL()加入v1不为NULL，则IFNULL()的返回值为v1，否则返回值为v2。

b.1/0的结果为NULL

mysql> SELECT IFNULL(1,2),IFNULL(NULL,0),IFNULL(1/0,'wrong');

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

| IFNULL(1,2) | IFNULL(NULL,0) | IFNULL(1/0,'wrong') |

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

| 1 | 0 | wrong |

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

1 row in set (0.00 sec)

mysql> SELECT 1/0;

+------+

| 1/0 |

+------+

| NULL |

+------+

1 row in set (0.00 sec)

4.3CASE函数

PS：CASE expr WHEN v1 THEN r1 WHEN V2 THEN r2 ELSE rn END

该函数表示，如果expr值等于某个vn，则返回对应位置THEN后面的结果，如果与所有值都不相等，则返回ELSE后面的rn

CASE WHEN v1 THEN r1 WHEN V2 THEN r2 ELSE rn END

该函数表示，某个vn值为TRUE时，返回对应位置THEN后面的结果，如果所有值都不为TRUE，则返回ELSE后面的rn

mysql> SELECT CASE 'zm' WHEN 'lly' THEN 'chihuo' WHEN 'jzj' THEN 'haok' WHEN 'zm' THEN 'zuibangde' ELSE 'fw' END;

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

| CASE 'zm' WHEN 'lly' THEN 'chihuo' WHEN 'jzj' THEN 'haok' WHEN 'zm' THEN 'zuibangde' ELSE 'fw' END |

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

| zuibangde |

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

1 row in set (0.00 sec)

mysql> SELECT CASE WHEN 1<0 THEN 'true' ELSE 'false' END ;

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

| CASE WHEN 1<0 THEN 'true' ELSE 'false' END |

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

| false |

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

1 row in set (0.00 sec)

五、系统信息函数

5.1获取MySQL版本号的函数VERSION();

mysql> SELECT VERSION();

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

| VERSION() |

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

| 5.6.36 |

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

1 row in set (0.00 sec)

5.2获取MySQL服务器当前连接的次数CONNECTION_ID()

PS:SHOW PROCESSLIST;SHOW FULL PROCESSLIST;

processlist 命令的输出结果显示了有哪些线程在运行，不仅可以查看当前所有的连接数，还可以查看当前的

连接状态，帮助识别出有问题的查询语句等。

如果是root账号，能看到所有用户的当前连接，如果是其他普通账号，则只能看到自己占用的连接。

show processlist;只列出前100条，如果想全部列出可使用show full processlist命令。

mysql> SELECT CONNECTION_ID();

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

| CONNECTION_ID() |

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

| 6 |

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

1 row in set (0.00 sec)

5.3查看当前使用的数据库DATABASE(),SCHEMA();

PS:DATABASE(),SCHEMA();意义相同

mysql> SELECT DATABASE(),SCHEMA();

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

| DATABASE() | SCHEMA() |

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

| mysql | mysql |

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

1 row in set (0.00 sec)

5.4获取用户名的函数USER(),CURRENT_USER,CURRENT_USER(),SYSTEM_USER(),SESSION_USER()

mysql> SELECT USER(),CURRENT_USER(),SYSTEM_USER();

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

| USER() | CURRENT_USER() | SYSTEM_USER() |

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

| root@localhost | root@localhost | root@localhost |

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

1 row in set (0.00 sec)

5.5获取字符串的字符集的函数CHARSET(str)

mysql> SELECT CHARSET('abc'),CHARSET(CONVERT('abc' USING latin1)),CHARSET(VERSION());

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

| CHARSET('abc') | CHARSET(CONVERT('abc' USING latin1)) | CHARSET(VERSION()) |

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

| utf8 | latin1 | utf8 |

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

1 row in set (0.00 sec)

5.6返回字符串的字符排列方式的函数COLLATION(str)

mysql> SELECT COLLATION(_latin2 'abc'),COLLATION(CONVERT('abc' USING utf8));

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

| COLLATION(_latin2 'abc') | COLLATION(CONVERT('abc' USING utf8)) |

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

| latin2_general_ci | utf8_general_ci |

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

1 row in set (0.00 sec)

5.7获取最后一个自动生成的ID值的函数LAST_INSERT_ID()

mysql> CREATE TABLE worker(Id INT AUTO_INCREMENT NOT NULL PRIMARY KEY,Name VARCHAR(30));

Query OK, 0 rows affected (0.40 sec)

mysql> INSERT INTO worker VALUES(NULL,'zm');

Query OK, 1 row affected (0.04 sec)

mysql> INSERT INTO worker VALUES(NULL,'jzj');

Query OK, 1 row affected (0.31 sec)

mysql> INSERT INTO worker VALUES(NULL,'lly');

Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO worker VALUES(NULL,'fw');

Query OK, 1 row affected (0.02 sec)

mysql> SELECT * FROM worker;

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

| Id | Name |

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

| 1 | zm |

| 2 | jzj |

| 3 | lly |

| 4 | fw |

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

4 rows in set (0.00 sec)

mysql> SELECT LAST_INSERT_ID();

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

| LAST_INSERT_ID() |

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

| 4 |

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

1 row in set (0.27 sec)

六、加密函数

6.1加密函数PASSWORD(str)

PS:MySQL将PASSWORD函数加密后的密码保存到用户权限表中

mysql> SELECT PASSWORD('helloworld');

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

| PASSWORD('helloworld') |

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

| *D35DB127DB631E6E27C6B75E8D376B04F64FAF83 |

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

1 row in set (0.00 sec)

6.2加密函数MD5(str)

mysql> SELECT MD5('helloworld');

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

| MD5('helloworld') |

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

| fc5e038d38a57032085441e7fe7010b0 |

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

1 row in set (0.00 sec)

6.3加密函数ENCODE(str,pswd_str)

mysql> SELECT ENCODE('select','drop'),LENGTH(ENCODE('select','drop'));

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

| ENCODE('select','drop') | LENGTH(ENCODE('select','drop')) |

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

| ̒잠 | 6 |

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

1 row in set (0.00 sec)

6.4解密函数DECODE(crypt_tr,pswd_str)

mysql> select decode(encode('select','drop'),'drop');

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

| decode(encode('select','drop'),'drop') |

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

| select |

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

1 row in set (0.00 sec)

七、其他函数

7.1格式化函数FORMAT(x,n)

mysql> SELECT FORMAT(123.123,2),FORMAT(1234.1,3),FORMAT(123,0);

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

| FORMAT(123.123,2) | FORMAT(1234.1,3) | FORMAT(123,0) |

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

| 123.12 | 1,234.100 | 123 |

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

1 row in set (0.00 sec)

7.2不同进制的数字进行转换的函数CONV(N,from_base,to_base)

mysql> SELECT CONV('A',16,2),CONV(10,10,2),CONV(10,10,8),CONV(10,10,16);

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

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

| 1010 | 1010 | 12 | A |

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

1 row in set (0.00 sec)

7.3IP地址转换为数字的函数INET_ATON(expr)

mysql> SELECT INET_ATON('192.168.1.1');

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

| INET_ATON('192.168.1.1') |

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

| 3232235777 |

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

1 row in set (0.27 sec)

7.3将数值网络地址转换成字符串网络地址的函数INET_NTOA()

mysql> SELECT INET_NTOA(3232235777);

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

| INET_NTOA(3232235777) |

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

| 192.168.1.1 |

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

1 row in set (0.00 sec)

7.4改变数据类型的函数CAST(x, AS type) 和CONVERT(x,type)

mysql> SELECT CAST(100 AS CHAR(2)),CONVERT('2017-07-16 22:11:30' ,TIME);

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

| CAST(100 AS CHAR(2)) | CONVERT('2017-07-16 22:11:30' ,TIME) |

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

| 10 | 22:11:30 |

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

1 row in set, 1 warning (0.00 sec)

7.5改变字符集的函数CONVERT(...USING...)

mysql> SELECT CHARSET('string'),CHARSET(CONVERT('string' USING latin1));

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

| CHARSET('string') | CHARSET(CONVERT('string' USING latin1)) |

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

| utf8 | latin1 |

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

1 row in set (0.00 sec)

7.6重复执行指定操作的函数BENCHMARK(count,expr)

mysql> SELECT PASSWORD('helloworld');

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

| PASSWORD('helloworld') |

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

| *D35DB127DB631E6E27C6B75E8D376B04F64FAF83 |

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

1 row in set (0.00 sec)

mysql> SELECT BENCHMARK(500000,PASSWORD('helloworld'));

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

| BENCHMARK(500000,PASSWORD('helloworld')) |

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

| 0 |

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

1 row in set (0.08 sec)

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