Using group by and having clause

Question

Using the following schema:

Supplier (sid, name, status, city)
Part (pid, name, color, weight, city)
Project (jid, name, city)
Supplies (sid, pid, jid**, qua         


        

Answer 1

First of all, you should use the JOIN syntax rather than FROM table1, table2, and you should always limit the grouping to as little fields as you need.

Altought I haven't tested, your first query seems fine to me, but could be re-written as:

SELECT s.sid, s.name
FROM 
    Supplier s
    INNER JOIN (
       SELECT su.sid
       FROM Supplies su
       GROUP BY su.sid
       HAVING COUNT(DISTINCT su.jid) > 1
    ) g
        ON g.sid = s.sid

Or simplified as:

SELECT sid, name
FROM Supplier s
WHERE (
    SELECT COUNT(DISTINCT su.jid)
    FROM Supplies su
    WHERE su.sid = s.sid
) > 1

However, your second query seems wrong to me, because you should also GROUP BY pid.

 SELECT s.sid, s.name
    FROM 
        Supplier s
        INNER JOIN (
            SELECT su.sid
            FROM Supplies su
            GROUP BY su.sid, su.pid
            HAVING COUNT(DISTINCT su.jid) > 1
        ) g
            ON g.sid = s.sid

As you may have noticed in the query above, I used the INNER JOIN syntax to perform the filtering, however it can be also written as:

SELECT s.sid, s.name
FROM Supplier s
WHERE (
     SELECT COUNT(DISTINCT su.jid)
     FROM Supplies su
     WHERE su.sid = s.sid
     GROUP BY su.sid, su.pid
) > 1