mysql时间相减的问题_MySQL

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mysql时间相减的问题

今天看到宁青同学的一条微博，提到mysql日期相减的错误结果，以前没有怎么注意，于是测试了一下，发现确实很坑爹，很容易踩雷，于是整理博客提醒一下大家。

先看一下错误的现象如下，第1条正确，第2，3条的t2-t1不正确：

[sql]

mysql> select t1,t2,t2-t1 from mytest;

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

| t1 | t2 | t2-t1 |

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

| 2013-04-21 16:59:33 | 2013-04-21 16:59:43 | 10 |

| 2013-04-21 16:59:33 | 2013-04-21 17:00:33 | 4100 |

| 2013-04-21 16:59:33 | 2013-04-21 17:59:35 | 10002 |

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

3 rows in set

全部测试脚本如下：

[sql]

--创建表

mysql> CREATE TABLE mytest (

t1 datetime,

t2 datetime

);

Query OK, 0 rows affected

--插入测试记录

mysql> insert into mytest(t1,t2) values('2013-04-21 16:59:33','2013-04-21 16:59:43');

Query OK, 1 row affected

mysql> insert into mytest(t1,t2) values('2013-04-21 16:59:33','2013-04-21 17:00:33');

Query OK, 1 row affected

mysql> insert into mytest(t1,t2) values('2013-04-21 16:59:33','2013-04-21 17:59:35');

Query OK, 1 row affected

--验证结果

mysql> select t1,t2,t2-t1 from mytest;

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

| t1 | t2 | t2-t1 |

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

| 2013-04-21 16:59:33 | 2013-04-21 16:59:43 | 10 |

| 2013-04-21 16:59:33 | 2013-04-21 17:00:33 | 4100 |

| 2013-04-21 16:59:33 | 2013-04-21 17:59:35 | 10002 |

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

3 rows in set

实际是mysql的时间相减是做了一个隐式转换操作，将时间转换为整数，但并不是用unix_timestamp转换，而是直接把年月日时分秒拼起来，如2013-04-21 16:59:33 直接转换为20130421165933，由于时间不是十进制，所以最后得到的结果没有意义，这也是导致上面出现坑爹的结果。

[sql]

mysql> select t1,

t2,

convert(t1, UNSIGNED INTEGER) ct1,

convert(t2, UNSIGNED INTEGER) ct2,

t2-t1,

convert(t2, UNSIGNED INTEGER) -convert(t1, UNSIGNED INTEGER) diff0

from mytest;

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

|t1 |t2 |ct1 |ct2 |t2-t1|diff0|

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

|2013-04-21 16:59:33|2013-04-21 16:59:43|20130421165933|20130421165943| 10| 10|

|2013-04-21 16:59:33|2013-04-21 17:00:33|20130421165933|20130421170033| 4100| 4100|

|2013-04-21 16:59:33|2013-04-21 17:59:35|20130421165933|20130421175935|10002|10002|

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

3 rows in set

要得到正确的时间相减秒值，有以下3种方法：

1、time_to_sec(timediff(t2, t1)),

2、timestampdiff(second, t1, t2),

3、unix_timestamp(t2) -unix_timestamp(t1)

[sql]

--测试脚本

mysql> select t1,

t2,

t2-t1,

time_to_sec(timediff(t2, t1)) diff1,

timestampdiff(second, t1, t2) diff2,

unix_timestamp(t2) -unix_timestamp(t1) diff3

from mytest;

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

| t1 | t2 | t2-t1 | diff1 | diff2 | diff3 |

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

| 2013-04-21 16:59:33 | 2013-04-21 16:59:43 | 10 | 10 | 10 | 10 |

| 2013-04-21 16:59:33 | 2013-04-21 17:00:33 | 4100 | 60 | 60 | 60 |

| 2013-04-21 16:59:33 | 2013-04-21 17:59:35 | 10002 | 3602 | 3602 | 3602 |

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

3 rows in set

这个问题2003年就有人在mysql4.0的版本时反馈，但mysql官方并不认为是bug，因为他们认为mysql并不支持时间直接相减操作，应该用专用函数处理，所以一直没有修正。但我认为这个很容易导致使用错误，要么就直接报错，要么显示正确的结果。

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