Deleting Records

问题

I have a table [user_logs] with the following fields [username], [datetimelog]

Sample Data

==============
user1   2011-06-28 08:49:01
user2   2011-06-28 08:59:38
user3   2011-06-28 09:04:31
user4   2011-06-28 10:00:15
user2   2011-06-28 10:28:54
user1   2011-06-29 08:31:22
user9   2011-06-29 08:32:32
user2   2011-06-29 10:13:53
user1   2011-06-29 13:11:15

I want to know how to create an SQL Delete query to delete all user logs EXCEPT their last log so that the above example will produce the following after a DELETE query

user1   2011-06-29 13:11:15
user2   2011-06-29 10:13:53
user3   2011-06-28 09:04:31
user4   2011-06-28 10:00:15
user9   2011-06-29 08:32:32

回答1:

What about:

DELETE FROM 
    MY_TABLE M -- delete from the table
LEFT JOIN
    MY_TABLE M2 ON M.user = M2.user -- table needs to be joined TO ITSELF
WHERE
    NOT M.LOG_DATE = MAX( M2.LOG_DATE ) -- Anything which isn't MAX goes.

Could that work?

回答2:

DELETE FROM table a WHERE time != (SELECT MAX(time) FROM table b WHERE b.user=a.user);

Here delete a row, if its not the maximum time in group with the same user_id

回答3:

DELETE from user_logs UL1, user_logs UL2
where UL1.username =UL2.datetimelog
and UL1.datetimelog < UL2.datetimelog

Try that

来源：https://stackoverflow.com/questions/6517664/deleting-records