使用CASE和INNER JOIN的MySQL查询错误

Question

I have a table with user info called 'a' and a table with data from different API's (twitter,foursquare) which in the example, based on the value of the a.api_type should become b. What I want in the end is to be able to grab the right avatar from the active API (either api_foursquare or api_twitter).

I have been trying to get this to work with this query for a while, but I keep getting this error. Sql is not my strongest point, so any tips on how to fix this would be great :)

  "[Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near

  INNER JOIN b ON b.user_id = a.user_id  at line 11"

SELECT a.user_id, 
       a.api_type, 
       b.avatar, 
       b.user_id
(CASE
       WHEN a.api_type = 0 THEN api_foursquare
       WHEN a.api_type = 1 THEN api_twitter
END) as b
FROM a WHERE a.cookie_hash = :cookie_hash
INNER JOIN b ON b.user_id = a.user_id

Answer 1

You cannot pick a table to join in a case statement. You should left-join to both tables, and then pick the value from one of them in the case statement, like this:

SELECT a.user_id, 
       a.api_type,
       (case WHEN a.api_type = 0 THEN b.avatar ELSE c.avatar END) as avatar,
       (case WHEN a.api_type = 0 THEN b.user_id ELSE c.user_id END) as user_id
FROM a 
  LEFT OUTER JOIN api_foursquare b ON b.user_id = a.user_id
  LEFT OUTER JOIN api_twitter c ON c.user_id = a.user_id
WHERE a.cookie_hash = :cookie_hash

You probably do not need the last expression (the ...as user_id one), because it is going to be equal to a.user_id if there is a row in either api_twitter or api_foursquare that matches a.user_id .

You also have to put the WHERE clause after the FROM clause:

EDIT: Taking into account ypercube's great suggestion, the query would look like this:

SELECT a.user_id, 
       a.api_type,
       COALESCE(b.avatar, c.avatar) as avatar
FROM a 
  LEFT OUTER JOIN api_foursquare b ON b.user_id = a.user_id AND a.api_type = 0
  LEFT OUTER JOIN api_twitter c ON c.user_id = a.user_id and a.api_type = 1
WHERE a.cookie_hash = :cookie_hash

Answer 2

You need comma after b.user_id and before (CASE :

SELECT a.user_id, 
    a.api_type, 
    b.avatar, 
    b.user_id,
    (CASE
           WHEN a.api_type = 0 THEN api_foursquare
           WHEN a.api_type = 1 THEN api_twitter
    END) as b
FROM a WHERE a.cookie_hash = :cookie_hash
    INNER JOIN b ON b.user_id = a.user_id