Question

If i remove USING (nid,sid), i get the column is ambigious error. I don’t quite understand what USING does here? Is it joining the nid and sid returned from the subquery to the nid, sid in the outer select query?

 SELECT sid, nid, max_rating FROM outcomes
JOIN (SELECT nid, sid, MAX(rating) AS max_rating from outcomes group by nid, sid) AS newtable
USING (nid, sid)
WHERE rating = max_rating AND name LIKE '%test%'

Answer 1

Yes, USING is being used to join the two tables using both nid and sid. But it has added advantages.

• First, your result will not have duplicate columns for nid and sid, as the USING statement merges the columns together.
• Second, if you would like to use nid or sid in your WHERE clause, you would not need to specify which table to use for that column, you could directly use something like sid > 10.