mysql時間相減的問題(bug)
今天看到寧青同學的一條微博,提到MySQL日期相減的錯誤結果,以前沒有怎麼注意,於是測試了一下,發現確實很坑爹,很容易踩雷,於是整理博客提醒一下大家。
先看一下錯誤的現象如下,第1條正確,第2,3條的t2-t1不正確:
- 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
全部測試腳本如下:
- --創建表
- mysql> CREATE TABLE mytest (
- t1 datetime,
- t2 datetime
- );
- Query OK, 0 rows affected
- --插入測試記錄
- 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
- --驗證結果
- 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
實際是mysql的時間相減是做了一個隱式轉換操作,將時間轉換爲整數,但並不是用unix_timestamp轉換,而是直接把年月日時分秒拼起來,如2013-04-21 16:59:33 直接轉換爲20130421165933,由於時間不是十進制,所以最後得到的結果沒有意義,這也是導致上面出現坑爹的結果。
- 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
要得到正確的時間相減秒值,有以下3種方法:
1、time_to_sec(timediff(t2, t1)),
2、timestampdiff(second, t1, t2),
3、unix_timestamp(t2) -unix_timestamp(t1)
- --測試腳本
- 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
這個問題2003年就有人在mysql4.0的版本時反饋,但mysql官方並不認爲是bug,因爲他們認爲mysql並不支持時間直接相減操作,應該用專用函數處理,所以一直沒有修正。但我認爲這個很容易導致使用錯誤,要麼就直接報錯,要麼顯示正確的結果。