mysql时间相减的问题(bug)

mysql时间相减的问题(bug)

    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：

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：

--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。

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)

--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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