I am using MySQL. Here is my schema:
Suppliers(sid: integer, sname: string, address string)
Parts(pid: integer, pname: string, color: string)
Catalog(sid: integer, pid: integer, cost: real)
(primary keys are bolded)
I am trying to write a query to select all parts that are made by at least two suppliers:
-- Find the pids of parts supplied by at least two different suppliers.
SELECT c1.pid -- select the pid
FROM Catalog AS c1 -- from the Catalog table
WHERE c1.pid IN ( -- where that pid is in the set:
SELECT c2.pid -- of pids
FROM Catalog AS c2 -- from catalog
WHERE c2.pid = c1.pid AND COUNT(c2.sid) >= 2 -- where there are at least two corresponding sids
1111 - Invalid use of group function
You need to use HAVING, not WHERE.
The difference is: the WHERE clause filters which rows MySQL selects. Then MySQL groups the rows together and aggregates the numbers for your COUNT function.
HAVING is like WHERE, only it happens after the COUNT value has been computed, so it'll work as you expect. Rewrite your subquery as:
( -- where that pid is in the set: SELECT c2.pid -- of pids FROM Catalog AS c2 -- from catalog WHERE c2.pid = c1.pid HAVING COUNT(c2.sid) >= 2)