SQL make a list of not existing objects - sql

Question

I have following problem.
Two tables. Product Table and Inventory Table.
In Product table are a list of all possible products and in Inventory the current stock of each store
e.g.
Inventory Table
ProductID Stock StoreID
1 10 1
2 10 1
3 10 1
1 10 2
Product Table
ProductID Product
1 Bananas
2 Apples
3 Oranges
4 Kiwi
What I want is a list of products that are not in stock for the stores.
Following the example the desired result would be
Store ProductID Product
1 4 Kiwi
2 2 Apples
2 3 Oranges
2 4 Kiwi
Now I tried several approaches from left joins, not in and not exist but haven't found a solution.
E.g.
SELECT *
FROM Inventory t1
left join Product t2 ON t2.ProductID = t1.ProductID
WHERE t2.ProductID IS NULL
But this returns nothing
Any help please.
Thank you

Answer 1

Solution to your problem:
FOR MySQL as well as MSSQL
SELECT i.storeId,p.ProductId, p.product
FROM Product p
CROSS JOIN (
SELECT Distinct storeId
FROM Inventory) i
LEFT JOIN Inventory iv
ON p.productID = iv.productId AND i.storeId = iv.storeId
WHERE iv.storeId IS NULL;
OUTPUT:
storeId ProductId product
1 4 Kiwi
2 2 Apples
2 3 Oranges
2 4 Kiwi
Follow the link to the demo:
http://sqlfiddle.com/#!9/71a854/9
CROSS JOIN:
The CROSS JOIN produced a result set which is the product of rows of two associated tables when no WHERE clause is used with CROSS JOIN.
In this join, the result set appeared by multiplying each row of the first table with all rows in the second table if no condition introduced with CROSS JOIN.
This kind of result is called as Cartesian Product.
Below picture will give you a more clear picture:
Source:https://www.w3resource.com/mysql/advance-query-in-mysql/mysql-cross-join.php

Answer 2

Your approach should work, although selecting columns from only the first table makes sense. And the first table should be the product table:
SELECT p.*
FROM Product p LEFT JOIN
Inventory i
ON i.ProductID = p.ProductID
WHERE i.ProductID IS NULL;
Your version returns all inventory products that are not in the Product table. That should not be possible, if your data is correctly formatted.
Another way to write the query uses NOT EXISTS:
select p.*
from product p
where not exists (select 1 from inventory i where i.productid = p.productid);
This version (which should have very good performance if you have an index on inventory(productid)) comes closer to how you expressed the question.