MySQL-常用函数
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一、操作符优先级
展示了所有操作符的执行优先级，从高到低，同一行中的操作符优先级相同，相同优先级的情况下则从左到右执行
如果想改变优先级执行顺序，则可以使用括号：
mysql> SELECT 1+2*3; #7 mysql> SELECT (1+2)*3;#9
二、对比操作符
#对比操作符的执行结果为true,false,null三种 Between A and B #代表检查值是否在A和B之间 Coalesce() #代表返回第一个非Null的值 = #代表相等操作符 > #代表大于操作符 >= #代表大于等于操作符 Greatest() #代表返回最大的值 In() #代表检查值是否在一系列的值之中 Interval() #代表返回比第一个参数小的参数的位置 is/is not #代表检查值是否与布尔值相同/不同 Is not null #代表检查值是否是非NULL Is null #代表检查值是否是NULL Isnull() #代表检查参数是NULL Least() #代表返回最小的参数 < #代表小于操作符 <= #代表小于等于操作符 Like #代表字符匹配 Not between A and B #代表检查值是否不在A和B的范围之内 !=/<> #代表不等于操作符 Not in() #代表检查值是否不在一系列值的当中 Not like #代表检查值是否不匹配 Strcmp() #对比两个字符串 ----------------------------------------------------------------------------------------- #=号对比操作符 mysql> SELECT 1 = 0; #0 mysql> SELECT \'0\' = 0; #1 mysql> SELECT \'0.0\' = 0; #1 mysql> SELECT \'0.01\' = 0; #0 mysql> SELECT \'.01\' = 0.01; #1 #<>/!=号对比操作符 mysql> SELECT \'.01\' <> \'0.01\'; #1 mysql> SELECT .01 <> \'0.01\'; #0 mysql> SELECT \'zapp\' <> \'zappp\'; #1 #<=小于等于对比操作符 mysql> SELECT 0.1 <= 2; #1 #<小于对比操作符 mysql> SELECT 2 < 2; #0 #>=大于等于对比操作符 mysql> SELECT 2 >= 2; #1 #>大于对比操作符 mysql> SELECT 2 > 2; #0 #is操作符 mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN; #1, 1, 1 #Is not操作符 mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN; #1, 1, 0 #Is null对比操作符 mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL; #0, 0, 1 #Is not null对比操作符 mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL; #1, 1, 0 #Expr between min and max对比操作符 #相当于min <= expr AND expr <= max mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEEN 3 and 1; #1, 0 mysql> SELECT 1 BETWEEN 2 AND 3; #0 mysql> SELECT \'b\' BETWEEN \'a\' AND \'c\'; #1 mysql> SELECT 2 BETWEEN 2 AND \'3\'; #1 mysql> SELECT 2 BETWEEN 2 AND \'x-3\'; #0 #expr NOT BETWEEN min AND max #相当于NOT (expr BETWEEN min AND max) #COALESCE(value,...)对比操作符 #返回第一个非NULL的值，如果没有非null值，则返回NULL mysql> SELECT COALESCE(NULL,2); #2 mysql> SELECT COALESCE(NULL,NULL,NULL); #NULL #GREATEST(value1,value2,...) #返回其中最大的值 mysql> SELECT GREATEST(2,0); #2 mysql> SELECT GREATEST(34.0,3.0,5.0,767.0); #767.0 mysql> SELECT GREATEST(\'B\',\'A\',\'C\'); #\'C\' #expr IN (value,...)对比操作符 #当expr值能在values中找到，则返回1，否则返回0 mysql> SELECT 2 IN (0,3,5,7); #0 mysql> SELECT \'wefwf\' IN (\'wee\',\'wefwf\',\'weg\'); #1 mysql> SELECT (3,4) IN ((1,2), (3,4)); #1 mysql> SELECT (3,4) IN ((1,2), (3,5)); #0 #expr NOT IN (value,...)对比操作符 #Isnull(expr)操作符 #如果expr是null，则返回1，否则返回0 mysql> SELECT ISNULL(1+1); #0 mysql> SELECT ISNULL(1/0); #1 #LEAST(value1,value2,...) #返回最小值，如果其中有值为null，则返回null mysql> SELECT LEAST(2,0); #0 mysql> SELECT LEAST(34.0,3.0,5.0,767.0); #3.0 mysql> SELECT LEAST(\'B\',\'A\',\'C\'); #\'A\'
三、逻辑操作符
逻辑操作符返回1 (TRUE), 0 (FALSE), 或者NULL
#NOT, !逻辑操作符代表非操作 mysql> SELECT NOT 10; #0 mysql> SELECT NOT 0; #1 mysql> SELECT NOT NULL; #NULL mysql> SELECT ! (1+1); #0 mysql> SELECT ! 1+1; #1 #And,&&逻辑操作符 mysql> SELECT 1 AND 1; #1 mysql> SELECT 1 AND 0; #0 mysql> SELECT 1 AND NULL; #NULL mysql> SELECT 0 AND NULL; #0 mysql> SELECT NULL AND 0; #0 #Or, ||逻辑操作符 mysql> SELECT 1 OR 1; #1 mysql> SELECT 1 OR 0; #1 mysql> SELECT 0 OR 0; #0 mysql> SELECT 0 OR NULL; #NULL mysql> SELECT 1 OR NULL; #1 #Xor逻辑操作符 mysql> SELECT 1 XOR 1; #0 mysql> SELECT 1 XOR 0; #1 mysql> SELECT 1 XOR NULL; #NULL mysql> SELECT 1 XOR 1 XOR 1; #1
四、分配操作符
分配操作符是指赋值操作
mysql> SELECT @var1, @var2; #NULL, NULL mysql> SELECT @var1 := 1, @var2; #1, NULL mysql> SELECT @var1, @var2; #1, NULL mysql> SELECT @var1, @var2 := @var1; #1, 1 mysql> SELECT @var1, @var2; #1, 1 mysql> SELECT @var1:=COUNT(*) FROM t1; #如果是4 mysql> SELECT @var1; #4 #":="操作符也可以用在update等语句 mysql> SELECT @var1; #4 mysql> SELECT * FROM t1; #1, 3, 5, 7 mysql> UPDATE t1 SET c1 = 2 WHERE c1 = @var1:= 1; mysql> SELECT @var1; #1 mysql> SELECT * FROM t1; #2, 3, 5, 7 #=操作符在两种情况下会被认为是赋值操作，而其他情况下会认为是对比操作符 #在set语句中，=操作符会被认为是赋值操作 mysql> set @a=1; mysql> select @a; #1 #在update语句中的set子句中，=操作符会被认为是赋值操作
五、流程控制函数
流程控制函数包含以下四种函数
#语法格式 CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END #当value等于compare_value时，则返回result，否则返回else里的result，如果没有else子句则返回null mysql> SELECT CASE 1 WHEN 1 THEN \'one\' WHEN 2 THEN \'two\' ELSE \'more\' END; #\'one\' #语法格式 CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END #当第一个condition满足时，则返回result，否则返回else里的result，如果没有else子句时则返回null mysql> SELECT CASE WHEN 1>0 THEN \'true\' ELSE \'false\' END; #\'true\' mysql> SELECT CASE BINARY \'B\' WHEN \'a\' THEN 1 WHEN \'b\' THEN 2 END; #NULL --------------------------------------------------------------------------------------------------------- IF(expr1,expr2,expr3) #当expr1为1/true时，则返回expr2，否则返回expr3 mysql> SELECT IF(1>2,2,3); #3 mysql> SELECT IF(1<2,\'yes\',\'no\'); #\'yes\' mysql> SELECT IF(STRCMP(\'test\',\'test1\'),\'no\',\'yes\'); #select STRCMP(\'test\',\'test1\');==>-1 #\'no\' --------------------------------------------------------------------------------------------------------- IFNULL(expr1,expr2) #当expr1为非null时，则返回expr1，否则返回expr2 mysql> SELECT IFNULL(1,0); #1 mysql> SELECT IFNULL(NULL,10); #10 mysql> SELECT IFNULL(1/0,10); #10 mysql> SELECT IFNULL(1/0,\'yes\'); #\'yes\' --------------------------------------------------------------------------------------------------------- NULLIF(expr1,expr2) #当expr1等于expr2时，则返回null，否则返回expr1 mysql> SELECT NULLIF(1,1); #NULL mysql> SELECT NULLIF(1,2); #1
补充：
select gender,if(gender=1,\'male\',\'female\') from Students; select sname,ifnull(sname,\'unknown\') from Students;
六、字符串函数
6.1、ASCII(str)
返回str字符串中最左边字符的ascii码值，如果是空串则返回0，如果str是null则返回null
mysql> SELECT ASCII(\'2\'); #50 mysql> SELECT ASCII(2); #50 mysql> SELECT ASCII(\'dx\'); #100 mysql> SELECT ASCII(\'d\'); #100
6.2、CHAR(N,... [USING charset_name])
将括号中的N转化成ascii码对应的字符，返回这些字符组成的字符串，其中的null会被忽略
mysql> SELECT CHAR(77,121,83,81,\'76\'); #\'MySQL\' -> \'MySQL\' mysql> SELECT CHAR(77,77.3,\'77.3\'); #\'MMM\' -> \'MMM‘ mysql> SELECT CHARSET(CHAR(X\'65\')), CHARSET(CHAR(X\'65\' USING utf8)); +----------------------+---------------------------------+ | CHARSET(CHAR(X\'65\')) | CHARSET(CHAR(X\'65\' USING utf8)) | +----------------------+---------------------------------+ | binary | utf8 | +----------------------+---------------------------------+
6.3、CHAR_LENGTH(str)
返回字符串的字符长度
mysql> select char_length(\'mysql\'); +----------------------+ | char_length(\'mysql\') | +----------------------+ | 5 | +----------------------+ 1 row in set (0.00 sec)
6.4、CONCAT(str1,str2,...) *****
返回括号里所有参数字符串连接在一起，当其中有参数为NULL时则返回NULL
mysql> SELECT CONCAT(\'My\', \'S\', \'QL\'); #\'MySQL\' mysql> SELECT CONCAT(\'My\', NULL, \'QL\'); #NULL mysql> SELECT CONCAT(14.3); #\'14.3\'
应用：如何提取名和姓？
mysql> select * from name; +-----+-----------+--------+---------+ | sid | sname | gender | dept_id | +-----+-----------+--------+---------+ | 1 | zhang san | 1 | 1 | | 2 | li si | 1 | 1 | | 3 | wang wu | 1 | 1 | +-----+-----------+--------+---------+ 3 rows in set (0.00 sec) mysql> select sname,substring(sname,1,locate(\' \',sname)-1) as firstname,substring(sname,locate(\' \',sname)+1,100) as lastname from name; +-----------+-----------+----------+ | sname | firstname | lastname | +-----------+-----------+----------+ | zhang san | zhang | san | | li si | li | si | | wang wu | wang | wu | +-----------+-----------+----------+ 3 rows in set (0.00 sec)
6.5、CONCAT_WS(separator,str1,str2,...)
返回以第一个参数为分隔符的连接后的一个字符串，当有参数为NULL时则null被忽略
mysql> SELECT CONCAT_WS(\',\',\'First name\',\'Second name\',\'Last Name\'); #\'First name,Second name,Last Name\' mysql> SELECT CONCAT_WS(\',\',\'First name\',NULL,\'Last Name\'); #\'First name,Last Name\'
6.6、INSERT(str,pos,len,newstr)
将str中从pos位置开始后的len个字符替换成newstr字符串
mysql> SELECT INSERT(\'Quadratic\', 3, 4, \'What\'); #\'QuWhattic\' mysql> SELECT INSERT(\'Quadratic\', -1, 4, \'What\'); #\'Quadratic\' mysql> SELECT INSERT(\'Quadratic\', 3, 100, \'What\'); #\'QuWhat\'
6.7、INSTR(str,substr)
返回str字符串中第一个出现substr字符串的位置
mysql> SELECT INSTR(\'foobarbar\', \'bar\'); #4 mysql> SELECT INSTR(\'xbar\', \'foobar\'); #0
6.8、LEFT(str,len)
返回str字符串中从左边开始的len个长度的字符
mysql> SELECT LEFT(\'foobarbar\', 5); #\'fooba\'
6.9、LENGTH(str)
返回str字符串的byte字节长度
mysql> SELECT LENGTH(\'text\'); #4
6.10、LOAD_FILE(file_name)
读取文件且返回文件内容为字符串
mysql> UPDATE t SET blob_col=LOAD_FILE(\'/tmp/picture\') WHERE id=1;
6.11、LOCATE(substr,str)
返回str字符串中第一次出现substr字符串的位置，如果没有则返回null
mysql> SELECT LOCATE(\'bar\', \'foobarbar\'); #4 mysql> SELECT LOCATE(\'xbar\', \'foobar\'); #0
6.12、LOCATE(substr,str,pos)
返回str字符串中从pos位置开始第一次出现substr字符串的位置，如果没有则返回null
mysql> SELECT LOCATE(\'bar\', \'foobarbar\', 5); #7
6.13、LOWER(str)
返回将str字符串中所有字符变换成小写后的字符串，但对二进制文本无效
mysql> SELECT LOWER(\'QUADRATICALLY\'); -> \'quadratically\' mysql> SET @str = BINARY \'New York\'; mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING latin1)); +-------------+-----------------------------------+ | LOWER(@str) | LOWER(CONVERT(@str USING latin1)) | +-------------+-----------------------------------+ | New York | new york | +-------------+-----------------------------------+
6.14、LPAD(str,len,padstr)
将str的左边补充为padstr，直到补充成len长度的字符串并返回；如果str的长度比len长，则返回str中最左边开始的len长度的字符
mysql> SELECT LPAD(\'hi\',4,\'??\'); #\'??hi\' mysql> SELECT LPAD(\'hi\',1,\'??\'); #\'h\'
6.15、LTRIM(str)
将str最左边的空格去掉并返回
mysql> SELECT LTRIM(\' barbar\'); #\'barbar\'
6.16、REPEAT(str,count)
将str重复count并组合成字符串返回，如果count<1，则返回空串
mysql> SELECT REPEAT(\'MySQL\', 3); #\'MySQLMySQLMySQL\'
6.17、REPLACE(str,from_str,to_str)
将所有str字符串中匹配from_str子串的地方都替换成to_str子字符串
mysql> SELECT REPLACE(\'www.mysql.com\', \'w\', \'Ww\'); #\'WwWwWw.mysql.com\'
6.18、REVERSE(str)
将str字符串中的字符按照倒序组合并返回
mysql> SELECT REVERSE(\'abc\'); #\'cba\'
6.19、RIGHT(str,len)
将str字符串中从右边开始的len个字符返回
mysql> SELECT RIGHT(\'foobarbar\', 4); #\'rbar\'
6.20、RPAD(str,len,padstr)
将字符串str从右边开始补充为padstr直到整体长度为len，如果str的长度本身大于len，则返回str中len长度的字符串
mysql> SELECT RPAD(\'hi\',5,\'?\'); #\'hi???\' mysql> SELECT RPAD(\'hi\',1,\'?\'); #\'h\'
6.21、RTRIM(str)
将字符串str右边的空格去掉并返回
mysql> SELECT RTRIM(\'barbar \'); #\'barbar\'
6.22、SPACE(N)
返回N个长度的空格组成的空字符串
mysql> SELECT SPACE(6);
6.23、SUBSTRING *****
SUBSTRING(str,pos), SUBSTRING(str FROM pos), SUBSTRING(str,pos,len), SUBSTRING(str FROM pos FOR len)
如果没有len参数，则返回从pos位置开始的str中的子字符串；如果有len参数，则从pos位置开始返回str中长度为len的子字符串；如果pos为负值，则代表pos从右边开始数
mysql> SELECT SUBSTRING(\'Quadratically\',5); #\'ratically\' mysql> SELECT SUBSTRING(\'foobarbar\' FROM 4); #\'barbar\' mysql> SELECT SUBSTRING(\'Quadratically\',5,6); #\'ratica\' mysql> SELECT SUBSTRING(\'Sakila\', -3); #\'ila\' mysql> SELECT SUBSTRING(\'Sakila\', -5, 3); #\'aki\' mysql> SELECT SUBSTRING(\'Sakila\' FROM -4 FOR 2); #\'ki\'
6.24、SUBSTRING_INDEX(str,delim,count)
当count为正数，则返回delim出现在str字符串中第count次之前的子字符串，如果是负数，则从右边开始计算
mysql> SELECT SUBSTRING_INDEX(\'www.mysql.com\', \'.\', 2); #\'www.mysql\' mysql> SELECT SUBSTRING_INDEX(\'www.mysql.com\', \'.\', -2); #\'mysql.com\'
6.25、UPPER(str)
返回将str字符串中所有字符转换成大写的字符串
mysql> SELECT UPPER(\'Hej\'); #\'HEJ\'
七、字符串对比函数
#通配符%表示匹配0个或多个字符 #通配符_表示匹配1个字符 mysql> SELECT \'David!\' LIKE \'David_\'; #1 mysql> SELECT \'David!\' LIKE \'%D%v%\'; #1 #当匹配字符中有特殊字符时，可以用 \\或者escape来指定特殊字符为字符 mysql> SELECT \'David!\' LIKE \'David\\_\'; #0 mysql> SELECT \'David_\' LIKE \'David\\_\'; #1 mysql> SELECT \'David_\' LIKE \'David|_\' ESCAPE \'|\'; #1 #expr NOT LIKE pat [ESCAPE \'escape_char\']是上一个字符串对比函数的反义 STRCMP(expr1,expr2) #当expr1等于expr2时等于0，当expr1小于expr2时为-1，反之为1 mysql> SELECT STRCMP(\'text\', \'text2\'); #-1 mysql> SELECT STRCMP(\'text2\', \'text\'); #1 mysql> SELECT STRCMP(\'text\', \'text\'); #0 mysql> SET @s1 = _latin1 \'x\' COLLATE latin1_general_ci; #大小写不敏感 mysql> SET @s2 = _latin1 \'X\' COLLATE latin1_general_ci; mysql> SET @s3 = _latin1 \'x\' COLLATE latin1_general_cs; #大小写敏感 mysql> SET @s4 = _latin1 \'X\' COLLATE latin1_general_cs; mysql> SELECT STRCMP(@s1, @s2), STRCMP(@s3, @s4); +------------------+------------------+ | STRCMP(@s1, @s2) | STRCMP(@s3, @s4) | +------------------+------------------+ | 0 | 1 | +------------------+------------------+
八、算数操作符
#/和DIV #/代表除法；div代表整数型除法，相除之后只取整数部分 mysql> SELECT 3/5; #0.60 mysql> SELECT 102/(1-1); #NULL mysql> SELECT 5 DIV 2, -5 DIV 2, 5 DIV -2, -5 DIV -2; #2, -2, -2, 2
九、数字函数
#ABS(X) 绝对值函数 mysql> SELECT ABS(2); #2 mysql> SELECT ABS(-32); #32 #CEILING(X)/CEIL(X) 返回>=X值的最小整数 mysql> SELECT CEILING(1.23); #2 mysql> SELECT CEILING(-1.23); #-1 #FLOOR(X) 返回<=X值的最大整数 mysql> SELECT FLOOR(1.23), FLOOR(-1.23); #1, -2 #MOD(N,M), N % M, N MOD M 取余数操作，返回两者相除之后剩下的余数 mysql> SELECT MOD(234, 10); #4 mysql> SELECT 253 % 7; #1 mysql> SELECT MOD(29,9); #2 mysql> SELECT 29 MOD 9; #2 mysql> SELECT MOD(34.5,3); #1.5 #RAND([N]) 获取0到1之间的随机小数，比如当想获取7~12之间的随机整数是可以使用 SELECT FLOOR(7 + (RAND() * 5)); Select * from students order by rand() limit 1; #ROUND(X), ROUND(X,D) 四舍五入为D位个小数，当D参数不存在是，则默认为0 mysql> SELECT ROUND(-1.23); #-1 mysql> SELECT ROUND(-1.58); #-2 mysql> SELECT ROUND(1.58); #2 mysql> SELECT ROUND(1.298, 1); #1.3 mysql> SELECT ROUND(1.298, 0); #1 mysql> SELECT ROUND(23.298, -1); #20 #TRUNCATE(X,D) 数字X只保留D位的小数，其余均舍弃 mysql> SELECT TRUNCATE(1.223,1); #1.2 mysql> SELECT TRUNCATE(1.999,1); #1.9 mysql> SELECT TRUNCATE(1.999,0); #1 mysql> SELECT TRUNCATE(-1.999,1); #-1.9 mysql> SELECT TRUNCATE(122,-2); #100 mysql> SELECT TRUNCATE(10.28*100,0); #1028
十、日期和时间函数
ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days) #当expr为正数时则为增加时间，为负数时则为减少时间 #Unit参数可以是任意时间单位 mysql> SELECT ADDDATE(\'2008-01-02\', INTERVAL 31 DAY); #\'2008-02-02\' mysql> SELECT ADDDATE(\'2008-01-02\', 31); #\'2008-02-02\' ADDTIME(expr1,expr2) #将expr2的时间增加到expr1上 mysql> SELECT ADDTIME(\'2007-12-31 23:59:59.999999\', \'1 1:1:1.000002\'); #\'2008-01-02 01:01:01.000001\' mysql> SELECT ADDTIME(\'01:00:00.999999\', \'02:00:00.999998\'); #\'03:00:01.999997\' CONVERT_TZ(dt,from_tz,to_tz) #将时间dt从from_tz这个时区转换成to_tz这个时区并返回 mysql> SELECT CONVERT_TZ(\'2004-01-01 12:00:00\',\'GMT\',\'MET\'); #\'2004-01-01 13:00:00\' mysql> SELECT CONVERT_TZ(\'2004-01-01 12:00:00\',\'+00:00\',\'+10:00\'); #\'2004-01-01 22:00:00\' CURDATE()， CURRENT_DATE, CURRENT_DATE() #返回以yyyy-mm-dd或者yyyymmdd格式的当前时间 mysql> SELECT CURDATE(); #\'2008-06-13\' mysql> SELECT CURDATE() + 0; #20080613 CURTIME(),CURRENT_TIME,CURRENT_TIME() #按照hh:mm:ss或者hhmmss格式返回当前时间 mysql> SELECT CURTIME(); #\'23:50:26\' mysql> SELECT CURTIME() + 0; #235026.000000 Now(), CURRENT_TIMESTAMP,CURRENT_TIMESTAMP() #返回当前的日期和时间，格式为yyyy-mm-dd hh:mi:ss或者yyyymmddhhmiss mysql> SELECT NOW(); #\'2007-12-15 23:50:26\' mysql> SELECT NOW() + 0; #20071215235026.000000 DATE(expr) #获取expr中的日期 mysql> SELECT DATE(\'2003-12-31 01:02:03\'); #\'2003-12-31\' DATEDIFF(expr1,expr2) #返回expr1和expr2之间的天数差异，忽略时分秒 mysql> SELECT DATEDIFF(\'2007-12-31 23:59:59\',\'2007-12-30\'); #1 mysql> SELECT DATEDIFF(\'2010-11-30 23:59:59\',\'2010-12-31\'); #-31 DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit) #增加或者减少时间 mysql> SELECT DATE_ADD(\'2000-12-31 23:59:59\',INTERVAL 1 SECOND); #\'2001-01-01 00:00:00\' mysql> SELECT DATE_ADD(\'2010-12-31 23:59:59\',INTERVAL 1 DAY); #\'2011-01-01 23:59:59\' mysql> SELECT DATE_ADD(\'2100-12-31 23:59:59\',INTERVAL \'1:1\' MINUTE_SECOND); #\'2101-01-01 00:01:00\' mysql> SELECT DATE_SUB(\'2005-01-01 00:00:00\',INTERVAL \'1 1:1:1\' DAY_SECOND); #\'2004-12-30 22:58:59\' mysql> SELECT DATE_ADD(\'1900-01-01 00:00:00\',INTERVAL \'-1 10\' DAY_HOUR); #\'1899-12-30 14:00:00\' mysql> SELECT DATE_SUB(\'1998-01-02\', INTERVAL 31 DAY); #\'1997-12-02\' mysql> SELECT DATE_ADD(\'1992-12-31 23:59:59.000002\',INTERVAL \'1.999999\' SECOND_MICROSECOND); #\'1993-01-01 00:00:01.000001\' mysql> SELECT DATE_ADD(\'2009-01-30\', INTERVAL 1 MONTH); #\'2009-02-28\' DATE_FORMAT(date,format) #将date日期时间转换成format格式 文档：https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_date-add mysql> SELECT DATE_FORMAT(\'2009-10-04 22:23:00\', \'%W %M %Y\'); #\'Sunday October 2009\' mysql> SELECT DATE_FORMAT(\'2007-10-04 22:23:00\', \'%H:%i:%s\'); #\'22:23:00\' mysql> SELECT DATE_FORMAT(\'1900-10-04 22:23:00\',\'%D %y %a %d %m %b %j\'); #\'4th 00 Thu 04 10 Oct 277\' mysql> SELECT DATE_FORMAT(\'1997-10-04 22:23:00\',\'%H %k %I %r %T %S %w\'); #\'22 22 10 10:23:00 PM 22:23:00 00 6\' mysql> SELECT DATE_FORMAT(\'1999-01-01\', \'%X %V\'); #\'1998 52\' mysql> SELECT DATE_FORMAT(\'2006-06-00\', \'%d\'); #\'00\' DAY(date), DAYOFMONTH(date) #返回date中日期在当前月份中是第几天 mysql> SELECT DAYOFMONTH(\'2007-02-03\'); #3 DAYNAME(date) #返回date时间是星期几 mysql> SELECT DAYNAME(\'2007-02-03\'); #\'Saturday’ DAYOFWEEK(date) #返回date时间是星期“几” mysql> SELECT DAYOFWEEK(\'2007-02-03\'); #7 DAYOFYEAR(date) #返回date是一年中的第几天，取值范围在1~366 mysql> SELECT DAYOFYEAR(\'2007-02-03\'); #34 EXTRACT(unit FROM date) #Unit单元和date_add/date_sub函数中的一样，是获取date日期的unit部分 mysql> SELECT EXTRACT(YEAR FROM \'2009-07-02\'); #2009 mysql> SELECT EXTRACT(YEAR_MONTH FROM \'2009-07-02 01:02:03\'); #200907 mysql> SELECT EXTRACT(DAY_MINUTE FROM \'2009-07-02 01:02:03\'); #20102 mysql> SELECT EXTRACT(MICROSECOND FROM \'2003-01-02 10:30:00.000123\'); #123 FROM_UNIXTIME(unix_timestamp), FROM_UNIXTIME(unix_timestamp,format) #根据给定的unixtime，返回yyyy-mm-dd hh:mi:ss或者yyyymmddhhmiss格式的具体时间，或者按照format返回时间 mysql> SELECT FROM_UNIXTIME(1447430881); #\'2015-11-13 10:08:01\' mysql> SELECT FROM_UNIXTIME(1447430881) + 0; #20151113100801 mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),\'%Y %D %M %h:%i:%s %x\'); #\'2015 13th November 10:08:01 2015\' LAST_DAY(date) #返回date日期所在月份的最后一天日期 mysql> SELECT LAST_DAY(\'2004-02-05\'); #\'2004-02-29\' mysql> SELECT LAST_DAY(\'2004-01-01 01:01:01\'); #\'2004-01-31\' mysql> SELECT LAST_DAY(\'2003-03-32\'); #NULL SYSDATE() #返回当前日期和时间，格式为yyyy-mm-dd hh:mi:ss或者yyyymmddhhmiss #和now()函数的区别在于now()返回的时间是语句执行的时间，而sysdate()返回的时间是该函数执行的时间 mysql> SELECT NOW(), SLEEP(2), NOW(); +---------------------+----------+---------------------+ | NOW() | SLEEP(2) | NOW() | +---------------------+----------+---------------------+ | 2019-10-06 22:41:52 | 0 | 2019-10-06 22:41:52 | +---------------------+----------+---------------------+ 1 row in set (2.00 sec) mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE(); +---------------------+----------+---------------------+ | SYSDATE() | SLEEP(2) | SYSDATE() | +---------------------+----------+---------------------+ | 2019-10-06 22:42:16 | 0 | 2019-10-06 22:42:18 | +---------------------+----------+---------------------+ 1 row in set (2.00 sec) TIME(expr) #返回expr日期时间中的时间部分 mysql> SELECT TIME(\'2003-12-31 01:02:03\'); #\'01:02:03\' mysql> SELECT TIME(\'2003-12-31 01:02:03.000123\'); #\'01:02:03.000123\' UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date) #如果没有date参数，则返回当前时间到1970-01-01 00:00:00之间的秒数，如果有date参数，则表示date到1970-01-01 00:00:00之间的秒数 mysql> SELECT UNIX_TIMESTAMP(); #1447431666 mysql> SELECT UNIX_TIMESTAMP(\'2015-11-13 10:20:19\'); #1447431619
十一、格式转换函数
#Cast()和convert()两个函数都可以用来转换数据类型或者转换字符集 #允许转换的数据类型包括： Binary[N] char[N] Date Datetime decimal[M,[D]] Time Signed [integer] Unsigned [integer] SELECT CONVERT(_latin1\'Müller\' USING utf8); SELECT CONVERT(\'test\', CHAR CHARACTER SET utf8); SELECT CAST(\'test\' AS CHAR CHARACTER SET utf8); select CAST(\'2000-01-01\' AS DATE); select CONVERT(\'2000-01-01\', DATE);
十二、聚合函数
#用在存在group by子句的语句中 AVG([DISTINCT] expr) #计算expr的平均值，distinct关键词表示是否排除重复值 mysql> SELECT student_name, AVG(test_score) FROM student GROUP BY student_name; COUNT(expr) #计算expr中的个数，如果没有匹配则返回0，注意NULL的区别 mysql> SELECT student.student_name,COUNT(*) FROM student,course WHERE student.student_id=course.student_id GROUP BY student_name; COUNT(DISTINCT expr,[expr...]) #计算有多少个不重复的expr值，注意是计算非NULL的个数 mysql> SELECT COUNT(DISTINCT results) FROM student; MAX([DISTINCT] expr),MIN([DISTINCT] expr) #返回expr中最大或者最小的值 mysql> SELECT student_name, MIN(test_score), MAX(test_score) FROM student GROUP BY student_name; SUM([DISTINCT] expr) #返回expr的求和值
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