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
);
First off, am I even going about this the right way?
Secondly, I get this error:
1111 - Invalid use of group function
What am I doing wrong?
This question is related to
mysql
sql
mysql-error-1111
First, the error you're getting is due to where you're using the COUNT
function -- you can't use an aggregate (or group) function in the WHERE
clause.
Second, instead of using a subquery, simply join the table to itself:
SELECT a.pid
FROM Catalog as a LEFT JOIN Catalog as b USING( pid )
WHERE a.sid != b.sid
GROUP BY a.pid
Which I believe should return only rows where at least two rows exist with the same pid
but there is are at least 2 sid
s. To make sure you get back only one row per pid
I've applied a grouping clause.
Source: Stackoverflow.com