Question

I’ve never figured out several syntax things about SQL subqueries. Mainly I’m interested in where in the parent query is it valid to place a subquery.

here’s an example which throws an error:

SELECT 
  sum(votes.vote) AS sum, 
  votes.vote IS NOT NULL AS did_i_vote, 
  purchase_id, subject, full_name 
FROM (
  SELECT vote FROM votes 
  where votes.acct_id=3 AND 
    votes.column_name='purchase_id'
) votes  
RIGHT JOIN items_purchased 
  ON votes.parent_id=items_purchased.purchase_id  
JOIN accounts 
  ON items_purchased.purchaser_account_id=accounts.acct_id   
JOIN items 
  ON items_purchased.item_id=items.folder_id   
WHERE purchase_id='2' 
GROUP BY items_purchased.purchase_id

How do I make this query work?

Answer 1

Your subquery must select every column that you wish to reference afterwards.

SELECT 
  sum(votes.vote) AS sum, 
  votes.vote IS NOT NULL AS did_i_vote, 
  purchase_id, subject, full_name 
FROM (
  SELECT vote, parent_id FROM votes 
  where votes.acct_id=3 AND 
    votes.column_name='purchase_id'
) votes  
RIGHT JOIN items_purchased 
  ON votes.parent_id=items_purchased.purchase_id  
JOIN accounts 
  ON items_purchased.purchaser_account_id=accounts.acct_id   
JOIN items 
  ON items_purchased.item_id=items.folder_id   
WHERE purchase_id='2' 
GROUP BY items_purchased.purchase_id

Is what I would assume (note that I select vote and parent_id in the subquery)