Overcoming ambiguous field error in SQL query

Question

I have 4 tables and this query

SELECT tag.id, title
FROM tag
LEFT JOIN tag_map ON ( tag.id = tag_map.tag_id ) 
LEFT JOIN post ON ( post.id = post_id ) 
LEFT JOIN         


        

Answer 1

Based on the additional information supplied in a comment to another response, it appears that the a DIFFERENT title field needs to be selected depending on the post_type, with post_types corresponding to the post, game, and video tables.

I'll assume that post_type is a one character text column.

You can do that in two ways, with LEFT JOIN or UNION.

SELECT tag.id, 
CASE post_type 
   WHEN 'P' THEN post.title 
   WHEN 'V' THEN video.title 
   WHEN 'G' THEN game.title
END CASE
FROM tag
LEFT JOIN tag_map ON ( tag.id = tag_map.tag_id ) 
LEFT JOIN post ON ( post.id = post_id ) 
LEFT JOIN game ON ( game.id = post_id ) 
LEFT JOIN video ON ( video.id = post_id ) 
WHERE tag_slug =  "blizzard"
ORDER BY tag_map.post_type

However, a UNION solution might be clearer:

SELECT tag.id, post_type, title 
  FROM tag_map INNER JOIN post ON tag_map.post_id = post.id
UNION ALL
SELECT tag.id, post_type, title 
  FROM tag_map INNER JOIN game ON tag_map.post_id = game.id
UNION ALL
SELECT tag.id, post_type, title 
  FROM tag_map INNER JOIN video ON tag_map.post_id = video.id
WHERE tag_slug =  "blizzard"
ORDER BY tag_map.post_type

(Note, I simplified the answers slightly because it's not clear to me the relation between tag and tag_map, you might need to introduce both tables into the SELECTS).