SQL: Ensuring all rows are checked against a set of rows - sql

Question

I have two tables that look like this:
Supplier
SN SNAME Stat City
+----------+-------+----+--------+
| S1 | Smith | 20 | London |
| S2 | Jones | 10 | Paris |
| S3 | Blake | 30 | Paris |
| S4 | Clark | 20 | London |
| S5 | Adams | 30 | Athens |
+----------+-------+----+--------+
Shipment
SN PN QTY
+----+----+-----+
| S1 | P1 | 300 |
| S1 | P2 | 200 |
| S1 | P3 | 400 |
| S1 | P4 | 200 |
| S1 | P5 | 100 |
| S1 | P6 | 100 |
| S2 | P1 | 300 |
| S2 | P2 | 400 |
| S3 | P2 | 200 |
| S4 | P2 | 200 |
| S4 | P4 | 300 |
| S4 | P5 | 400 |
+----+----+-----+
I have the find the following:
List the supplier names (SNAME) for those suppliers who ship at least all those parts (PN) supplied by supplier S2.
In other words, I need to list all those suppliers who ship at least P1 and P2, although obviously my query needs to be focused more on the question at hand.
I'm pretty sure I have to use some form of NOT EXISTS -- perhaps the double not exists, to achieve this. I tried doing a self-join of the Shipment table to itself -- but I can't understand how to get it beyond checking for ANY item that appears in the PN for S2 and instead check to make sure ALL parts for S2 are in the list before including the name in the result.

Answer 1

I used a Common Table Expression to accomplish this:
--CTE will contain records from the Supplier you are looking to check against
WITH desiredSN AS
(
SELECT s.SN, s.PN
FROM Shipment AS s
WHERE s.SN = 'S2'
)
--SELECT will join all other shippers with each record of your Supplier S2 here
SELECT s.SN
FROM Shipment AS s
INNER JOIN desiredSN AS d ON s.PN = d.PN
WHERE s.SN != 'S2'
GROUP BY s.SN
--Having ensures that the count of your old supplier matches
--the number a potential supplier overlaps with
HAVING COUNT(1) =
(
SELECT COUNT(1)
FROM desiredSN AS d
)
If you do not want to use the CTE, you can use subqueries instead:
SELECT s.SN
FROM Shipment AS s
INNER JOIN
(
SELECT s.SN, s.PN
FROM Shipment AS s
WHERE s.SN = 'S2'
)
AS d ON s.PN = d.PN
WHERE s.SN != 'S2'
GROUP BY s.SN
HAVING COUNT(1) =
(
SELECT COUNT(1)
FROM Shipment AS s
WHERE s.SN = 'S2'
)

Answer 2

Could you not use a nested select statement to refine your results?
For example:
SELECT SNAME FROM Supplier
INNER JOIN Shipment ON Supplier.SN = Shipment.SN
WHERE Shipment.PN IN (SELECT PN FROM Shipment WHERE SN = S2)
This way, the parts (PN) are restricted to just those that are sold by S2.
edited changed the statement to
Shipment.PN IN (SELECT PN FROM Shipment WHERE SN = S2)
Now, even if they sold one of everything, they'll only be returned if they sold at least what S2 sells.