Question

From my understanding sub queries will be called everytime a row is looked at and in this case every row will be looked at.

How do i rewrite this query? The subquery only needs to be ran once but i cant think of how to select id when i have to remove ids that only have 1 entry in the group (i want group count>1).

The purpose is to get a list of rows that have the same size as other rows

select id 
from file 
where size in 
    ( 
    select size from 
        (
        select count(*) as c, size 
        from file 
        group by size 
        having c>1 and size>0
        ) as t
    )

Answer 1

You could use just a subquery instead of two:

select id 
from file 
where size in (
    select size 
    from file 
    group by size 
    having count(*)>1 and size>0
    )

but if you want to use just joins, you could use something like this:

select distinct file.id
from file inner join file file1 on file.size = file1.size
where file.size>0 and file.id <> file1.id