MySQL日期和时间函数讲解 1. NOW、CURRENT_TIMESTAMP和SYSDATE 这些函数都能返回当前的系统时间,它们之间有区别吗?先来看个例子。 mysql?SELECT?NOW(),CURRENT_TIMESTAMP(),SYSDATE()\G; ***************************?1. row?*************************** NO
MySQL日期和时间函数讲解1. NOW、CURRENT_TIMESTAMP和SYSDATE
这些函数都能返回当前的系统时间,它们之间有区别吗?先来看个例子。
mysql>?SELECT?NOW(),CURRENT_TIMESTAMP(),SYSDATE()\G;***************************?1. row?***************************
NOW():?2011-02-04?20:35:04
CURRENT_TIMESTAMP():?2011-02-04?20:35:04
SYSDATE():?2011-02-04?20:35:04
从上面的例子看来,3个函数都是返回当前的系统时间,再来看下面这个例子:
mysql>SELECT->?????NOW(),
->???? CURRENT_TIMESTAMP(),
->???? SYSDATE(),
->???? SLEEP(2),
->?????NOW(),
->???? CURRENT_TIMESTAMP(),
->???? SYSDATE()\G
***************************?1. row?***************************
NOW():?2011-11-29?21:04:49
CURRENT_TIMESTAMP():?2011-11-29?21:04:49
SYSDATE():?2011-11-29?21:04:49
SLEEP(2):?0
NOW():?2011-11-29?21:04:49
CURRENT_TIMESTAMP():?2011-11-29?21:04:49
SYSDATE():?2011-11-29?21:04:51
1?row in?set?(2.00?sec)
在上面这个例子中人为地加入了SLEEP函数,让其等待2秒,这时可以发现SYSDATE返回的时间和NOW及CURRENT_TIMESTAMP是不同的,SYSDATE函数慢了2秒。究其原因是这3个函数有略微区别:
CURRENT_TIMESTAMP是NOW的同义词,也就是说两者是相同的。
SYSDATE函数返回的是执行到当前函数时的时间,而NOW返回的是执行SQL语句时的时间。
因此在上面的例子中,两次执行SYSDATE函数返回不同的时间是因为第二次调用执行该函数时等待了前面SLEEP函数2秒。而对于NOW函数,不管是在SLEEP函数之前还是之后执行,返回的都是执行这条SQL语句时的时间。
2.时间加减函数
先来看一个例子。
mysql>?SELECT?NOW(),NOW()+0\G;***************************?1. row?***************************
NOW():?2011-02-04?20:46:33
NOW()+0:?20110204204633.000000
1?row in?set?(0.00?sec)
可以看到,NOW()函数可以返回时间,也可以返回一个数字,就看用户如何使用。如果相对当前时间进行增加或减少,并不能直接加上或减去一个数字,而需要使用特定的函数,如DATE_ADD或DATE_SUB,前者表示增加,后者表示减少。其具体的使用方法有DATE_ADD(date,INTERVAL expr unit)和 DATE_SUB(date,INTERVAL expr unit),示例如下:
mysql>?SELECT?NOW()?AS?now,DATE_ADD(now(),INTERVAL?1?DAY)?AS?tomorrow,
DATE_SUB(now(),INTERVAL?1?DAY)?AS?yesterday\G;
***************************?1. row?***************************
now:?2011-02-04?20:53:25
tomorrow:?2011-02-05?20:53:25
yesterday:?2011-02-03?20:53:25
1?row in?set?(0.00?sec)
其中expr值可以是正值也可以是负值,因此可以使用DATE_ADD函数来完成DATE_SUB函数的工作,例如:
mysql>?SELECT?NOW()?AS?now,DATE_ADD(NOW(),INTERVAL?1?DAY)?AS?tomorrow,
DATE_ADD(NOW(),INTERVAL?-1?DAY)?AS?yesterday\G;
***************************?1. row?***************************
now:?2011-02-04?20:55:40
tomorrow:?2011-02-05?20:55:40
yesterday:?2011-02-03?20:55:40
1?row in?set?(0.00?sec)
还有一个问题,如果是闰月,那么DATE_ADD函数怎么处理呢?MySQL的默认行为是这样的:如果目标年份是闰月,那么返回的日期为2月29日;如果不是闰月,那么返回日期是2月28日。示例如下:
mysql>?SELECT?DATE_ADD(’2000-2-29′,INTERVAL 4 YEAR) AS year;+————+
|?year????????|
+————+
|?2004-02-29?|
+————+
1?row in?set?(0.00?sec)
?
mysql>?SELECT?DATE_ADD(’2000-2-29′,INTERVAL 1 YEAR) AS year;
+————+
|?year????????|
+————+
|?2001-02-28?|
+————+
1?row in?set?(0.00?sec)
在上面的例子中使用了DAY和YEAR数据类型,其实也可以使用MICROSECOND、SECOND、MINUTE、HOUR、WEEK、MONTH等类型,例如:
mysql>?SELECT?NOW()?AS?now,DATE_ADD(NOW(), INTERVAL?1?HOUR)?AS?next?time\G;
***************************?1. row?***************************
now:?2011-02-04?21:00:15
next?time:?2011-02-04?22:00:15
1?row in?set?(0.00?sec)
3. DATE_FORMAT函数
这个函数本身没有什么需要探讨的地方,其作用只是按照用户的需求格式化打印出日期,例如:
mysql>?SELECT?DATE_FORMAT(NOW(),’%Y%m%d’) AS datetime;+———-+
| datetime |
+———-+
|?20110204?|
+———-+
1?row in?set?(0.00?sec)
但是开发人员往往会错误地使用这个函数,导致非常严重的后果。例如在需要查询某一天的数据时,有些开发人员会写如下的语句:
SELECT * FROM table WHERE DATE_FORMAT(date,’%Y%m%d’)=’xxxx-xx-xx’;
一般来说表中都会有一个对日期类型的索引,如果使用上述的语句,优化器绝对不会使用索引,也不可能通过索引来查询数据,因此上述查询的执行效率可能非常低。
1 楼 hunan84229247 2013-10-08

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