Question

I have SQL table that holds buyers personal data such as FirstName, LastName, OGCard;
OGCard – field that indicates clients personal card number.

Next I have table called Cards which contains data about ten items that were purchased in the following format: OGCard, Item1ID,…,Item10ID,Item1Quantity,…,Item10Quantity

And in table Items I got such structure ItemID, ItemName

I want to get all purchases from CardNumber

I think i should do something like that

SELECT Buyers.FirstName, Items.Name, Cards.Item1Quantity
FROM Buyers
INNER JOIN Cards ON Buyers.OGCard=Cards.OGCard
INNER JOIN Items ON Cards.Item1ID=Items.ItemName
WHERE OGCard=13451

In this way I can see the name of first item and it’s quantity. However i cannot perform the same join to column Item2ID, and so on. Could you suggest me some solution?

Answer 1

It’s possible to join the same table twice by using aliases:

SELECT
    Buyers.FirstName,
    Items1.Name, Cards.Item1Quantity,
    Items2.Name, Cards.Item2Quantity,
    Items3.Name, Cards.Item3Quantity,
    Items4.Name, Cards.Item4Quantity
FROM Buyers
INNER JOIN Cards ON Buyers.OGCard=Cards.OGCard
INNER JOIN Items AS Items1 ON Cards.Item1ID=Items.ItemName
INNER JOIN Items AS Items2 ON Cards.Item2ID=Items.ItemName
INNER JOIN Items AS Items3 ON Cards.Item3ID=Items.ItemName
INNER JOIN Items AS Items4 ON Cards.Item4ID=Items.ItemName
WHERE OGCard=13451

If some of the Items_n_IDs can be missing, you could use a LEFT JOIN instead of an INNER JOIN so that you still get the results for the values that are present.

However I’d strongly recommend you to normalize your database rather than going with this approach.