从MYSQL删除数据

Question

I have a table which contains data as below in MySQL. Size shows file's size in MB.

--------------------------------------------
| id  | name | size | user_id | created_at |
--------------------------------------------
| 1   | a    | 120  | 1       | 2017-04-03 |
--------------------------------------------
| 2   | b    | 280  | 1       | 2017-04-04 |
--------------------------------------------
| 3   | c    | 220  | 1       | 2017-04-05 |
--------------------------------------------

Now, I want to delete the oldest data if a particular user has crossed the limit of 500MB. In above case, ID#1 needs to remove.

Can anyone please help me to write a query?

Answer 1

You can build a query for that in three steps. First, you get for each user the dates that have files loaded later the 500MB threshold:

select  t1.user_id, t1.created_at
from    files t1
join    files t2
on      t1.user_id = t2.user_id and
        t2.created_at > t1.created_at
group by t1.user_id, t1.created_at
having  sum(t2.size) >= 500

Then for each user you get the higher of those dates

select  user_id, max(created_at)
from    (
            select  t1.user_id, t1.created_at
            from    files t1
            join    files t2
            on      t1.user_id = t2.user_id and
                    t2.created_at > t1.created_at
            group by t1.user_id, t1.created_at
            having  sum(t2.size) >= 500
        )

Finally, you use that as reference for the actual delete , by joining it with the source table:

delete t1
from   yourTable t1
join   (
            select  user_id, max(created_at) as created_at
            from    (
                        select  t1.user_id, t1.created_at
                        from    files t1
                        join    files t2
                        on      t1.user_id = t2.user_id and
                                t2.created_at > t1.created_at
                        group by t1.user_id, t1.created_at
                        having  sum(t2.size) >= 500
                    )
       ) t2
on     t1.user_id = t2.user_id and
       t1.created_at = t2.created_at

Edit

Working Rextester