Using foreign key - sql

Question

I am new to foreign key, but I understand the concept very well.
I have found lot of documentation on how to create / delete them but not how to use them. My schema is as follows.
Stock table:
PartID | Model | Type | Vendor
------------------------------
1 | DDr2 | RAM | shop1
2 | DDr3 | RAM | shop1
3 | WD1 | HDD | shop2
4 | WD2 | HDD | shop2
Then product Table
ProdID | Name | PartID1 | PartID2 ...
1 | PC1 | 1 | 2
2 | PC1 | 3 | 4
How do I use select to get
| PC1 | DDr2 | DDR3 |
| PC1 | WD1 | WD2 |
with PartID2 and PartID3 foreign key linked to PartID primary key?

Answer 1

The concept of Foreign Keys is to link the IDs in one table to the lisk of unique IDs in another. In your example, you have unique parts with unique IDs and Products that can use those parts, so in your product table, you could have multiple part IDs being used in multiple rows.
Foreign Keys are used to keep referential integrity in your database, you can use joins to get the Data you want:
SELECT A.NAME,
B.Model,
C.Model
FROM PRODUCTS A
INNER JOIN PARTS B ON B.PARTID1 = A.PARTID
INNER JOIN PARTS C ON C.PARTID1 = A.PARTID
WHERE A.PRODID = 1

Answer 2

The short answer is you could do
select p.name, a.model as part1, b.model as part2, c.model as part3
from product p, stock a, stock b, stock c
where p.partid1 = a.partid and p.partid2 = b.partid and p.partid3 = c.partid
The longer answer is that this isn't really a good table design for what you're trying to do. It assumes that you always have a fixed number of parts for any item (or at least no more than some fixed number). A better design would be:
Part Table:
partID | model | type | vendor
Product Table:
productID | name
Product_Parts Table:
productID | partID
where productID in Product_parts is a foreign key into Product and partID is a foreign key into the Part table.

Answer 3

SELECT s1.Name, p1.Model, p2.Model FROM stock st
INNER JOIN product p1
ON st.PartID1 = p1.PartID1
INNER JOIN product p2
ON st.PartID2 = p2.PartID1
Take one JOIN at the time first join stock and parts table
then again join result of this join to parts table.
SQL parser will use parts table as two separate tables an so you can have two results from same tabe in single row.

Answer 4

you can join in a table more than once in the same sql statement. in this case, you need to join your stock table twice, once to get the name of each part in your product.
SELECT pr.ProdID, s1.Model, s2.Model
FROM Product pr, Stock s1, Stock s2
WHERE pr.PartID1 = s1.PartID
AND pr.PartID2 = s2.PartID

Answer 5

Using a LEFT OUTER JOIN means that the product will still be returned event if the Part1ID or Part2ID values are set to NULL.
SELECT P.Name,
S1.Model,
S2.Model
FROM Product P
LEFT OUTER JOIN Stock S1 ON P.PartID1 = S1.PartID
INNER JOIN Stock S2 ON P.PartID2 = S2.PartID