Question

these are the tables I have:

------    ----------     -------
|tags| -- |tagitems|  -- |items|
------    ----------     -------

so tags have a m:n relationship with items.

I want to get a list of all tags with an additional boolean column which is created on the fly and holds for each tag the boolean value whether the tag has a relation to an item id I specify or not.

So the desired result is:

------------------------------
|tag.id | is_assigned_to_item|
------------------------------
|   1   |          true      |
|   3   |          false     |
------------------------------

What could be a query for this using mysql?

Cheers

Sebastian

Answer 1

Use an outer join to relate the tag table to the tagitems table, including the relevant item_id you specify in the ON clause.

select tag.tag_id, 
  (count(tagitems.tag_id) > 0) as is_assigned_to_item
from tag 
  left outer join tagitems on tagitems.tag_id=tag.tag_id and tagitems.item_id = ?
group by tag.tag_id