Question

I got three tables

objects     id, field1, field2, ...
groups      id, title, color
group_ref   oid, gid

So every object can be in n groups.

I want to show a list of objects and one field of that list are the groups an object is in. So here is what i came up with:

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"), GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id 
WHERE 1 
GROUP BY o.id

That actually works and i can create the needed interface out of the (concatenated) groups field.

Problem is: How to select objects who are in one specific group only (e.g. gid=4)?

This only gets a part of the results:

WHERE gr.id = 4

or

HAVING gr.id = 4

Any help greatly appreciated! (maybe there is a more elegant way for the concat thingy too)

Answer 1

If I understand you correctly, you want to get all objects in group 4 and all the groups those object are in. You want to JOIN on the group_ref table twice, once to filter (regular JOIN) and once to get the related groups (LEFT JOIN).

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"),
GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
JOIN group_ref AS gfilter ON o.id = gfilter.oid AND gfilter.gid=4
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id 
WHERE 1 
GROUP BY o.id