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).

Answer 2

SELECT 
tag.id, 
tag_map.title as tagmaptitle,
post.title as posttitle,
game.title as gametitle,
video.title as viedotitle,
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

Answer 3

You need to specify which table you want title to come from. Taking a guess that title will be in game you would need to specify game.title.

If I were you I'd start looking into splitting this down a bit using temporary tables. Although theres nothing wrong with 4 joins it does tend to start getting confusing.

Example temp table:

CREATE TABLE #Yaks (
YakID int,
YakName char(30) )

In your scenario you would create a temp table and then fill it with an INSERT and UPDATE based on individual joins so at the end all you have to do is SELECT * FROM @temptable