Question

I have two tables: a Files table, which includes the file type, and a File Properties table, which references the file table via a foreign key. Sample Files table:

| id | name  | type |
---------------------
|  1 | file1 | zip  |
|  2 | file2 | zip  |
|  3 | file3 | zip  |
|  4 | file4 | jpg  |

And the Properties table:

| file_id  | property |
-----------------------
|  1       |     x    |
|  2       |     x    |

I want to make a query, which shows the count of each file type, and how many files of that type have a property.

So in the example, the result would be

| type | filecount  | prop count |
----------------------------------
| zip  |    3       |     2      |
| jpg  |    1       |     0      |

I could accomplish this by

select f.type, (select count(id) from files where type = f.type), count(fp.id) from 
files as f, file_properties as fp where f.id = fp.file_id group by f.type;

But this seems very suboptimal and is very slow. Any better way to do this?

Answer 1

select type, count(*) as filecount, sum(pc.count) as [prop count]
from Files f
left outer join (
    select file_id, count(*) as count
    from Properties p
    group by file_id
) pc on f.id = pc.file_id
group by type