Question

am having a problem constructing a query
here is simplified tables structure
3 tables

Event [Event_id , Event_name]

Event_files [Event_id(FK) , File_id(FK)]

Uploaded_Files[File_id , File_type, File_path]

we mainly have 2 file types

• image = 2

• document = 4

what am trying to do is to get the events along with their images (if they have an image )
am trying to do this with this Query

select e.id, e.name,uf.id as file_id,uf.path  
from event e
left join event_file ef on ef.event_id = e.id
left join uploaded_file uf ON ef.file_id = uf.id

i know that i need to apply a condition but each time i do in the where or ON there is always problem with the Query
for example if i apply :

left join uploaded_file uf ON ef.file_id = uf.id AND (uf.type = 2 )

it will still return 2 records for the events that has both image and file one of them with file_path null .

on the other hand if i do the following :

where (uf.id is null OR (uf.id is not null AND uf.type=2))

the events with only files and no image will not be returned any more

is there is solution please ?

thanks in advance

Answer 1

SELECT e.id, e.name, f.file_id AS file_id, f.path
FROM event e
LEFT JOIN
(
    SELECT ef.event_id, uf.id AS file_id, uf.path
    FROM event_file ef
    INNER JOIN uploaded_file uf ON ef.file_id = uf.id AND uf.type = 2
) f ON f.event_id = e.id

This should do (untested.)
The reason you’re getting the empty record is because you only specify the uf.type condition on the uploaded_file table, which imposes nothing on the left join for event_file.