Nick Heiner - 6 months ago 31

SQL Question

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?

Answer

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)
```