解读mysql时间函数sysdate()和now()_MySQL
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解读mysql时间函数sysdate()和now()
NOW()
返回当前日期和时间值,其格式为 'YYYY-MM-DD HH:MM:SS' 或YYYYMMDDHHMMSS , 具体格式取决于该函数是否用在字符串中或数字语境中。
mysql> SELECT NOW();
-> '1997-12-15 23:50:26'
mysql> SELECT NOW() + 0;
-> 19971215235026
在一个存储程序或触发器内, NOW() 返回一个常数时间,该常数指示了该程序或触发语句开始执行的时间。这同SYSDATE()的运行有所不同。
SYSDATE()
返回当前日期和时间值,格式为'YYYY-MM-DD HH:MM:SS' 或YYYYMMDDHHMMSS, 具体格式根据函数是否用在字符串或数字语境而定。
在一个存储程序或触发器中, SYSDATE()返回其执行的时间, 而非存储成都或触发语句开始执行的时间。这个NOW()的运作有所不同。
我的理解:
now()是应用向mysql服务器发起执行语句时(无论是发送的sql还是存储过程的名字),now()时间已经产生。
调用在该存储过程中或者sql中调用now(),即为调用该变量。
而sysdate()函数是每次实时去取的
在我们公司的业务中,由于禁止使用存储过程,显然无论使用sysdate()还是now()都能够满足业务需求。当然,考虑到sysdate要实时取时间,我认为now()是比较推荐的.
实践是检验真理的唯一标准:
1.存储过程:
BEGIN
select now(),sysdate();
select sleep(3) ;
select now(),sysdate();
END
执行结果:
mysql> call test;
+---------------------+---------------------+
| now() | sysdate() |
+---------------------+---------------------+
| 2011-01-21 11:22:58 | 2011-01-21 11:22:58 |
+---------------------+---------------------+
1 row in set (0.00 sec)
+----------+
| sleep(3) |
+----------+
| 0 |
+----------+
1 row in set (3.00 sec)
+---------------------+---------------------+
| now() | sysdate() |
+---------------------+---------------------+
| 2011-01-21 11:23:01 | 2011-01-21 11:23:01 |
+---------------------+---------------------+
1 row in set (3.01 sec)
Query OK, 0 rows affected (3.01 sec)
通过如上结果来看,第一个now()跟第二个now()执行结果不一致;对存储过程之内的now()函数,并非取自存储过程执行的开始时间。
2.sql语句测试:
mysql> select now(),sleep(3),now();
+---------------------+----------+---------------------+
| now() | sleep(3) | now() |
+---------------------+----------+---------------------+
| 2011-01-21 11:25:45 | 0 | 2011-01-21 11:25:45 |
+---------------------+----------+---------------------+
1 row in set (3.00 sec)
评:now()函数,每次取语句开始的执行时间.
mysql> select sysdate(),sleep(3),sysdate();
+---------------------+----------+---------------------+
| sysdate() | sleep(3) | sysdate() |
+---------------------+----------+---------------------+
| 2011-01-21 11:26:44 | 0 | 2011-01-21 11:26:47 |
+---------------------+----------+---------------------+
1 row in set (3.02 sec)
评:sysdate() 每次会取实际的时间。
综合上面的结果:
1.在公司绝大部分业务中now()函数已经够用,特别是gmt_create,gmt_modified字段的时间,无疑要用now()
2.sysdate()用在需要每个字段执行的时候实时取时间的。
3.mysql的字段是从左到右依次执行。
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