Return stock in a warehouse even if there is no row for that given stock - sql

Question

I have 5 different tables:
Toasters: product name (foreign key to products and primary key), slots, serial
Microwaves: product name (same as toaster), wattage
Products: product name (primary key)
Stock: product (fk to product), warehouse (fk to warehouse), amount
Warehouse: name (primary key)
toasters and microwaves are child tables of products (although its not using postgres inheritance, since there are issues with it). They represent different models of toasters (simplified to just slots and wattage here). Every toaster and microwave has exactly 1 entry in the products table.
Now the goal is to create a query that essentially gives me an amount of all products across all warehouses for a given list of product names. The problem is, that some warehouses may not have a stock entry for a certain product. They also have either one stock per product or none.
I have managed to make it work for a single warehouse:
--join together all 3 product tables and select all desired products
WITH selIProducts AS(
SELECT
--Get the products category by checking if the table is part of the query
(CASE
WHEN toasters IS NOT NULL THEN 'toasters'
WHEN microwaves IS NOT NULL THEN 'microwaves'
ELSE 'ERROR'
END) as category,
products.name as productName,
*
FROM products
--I need a full join to include everything
FULL JOIN toasters ON toasters.name=products.name
FULL JOIN microwaves ON microwaves.name=products.name
WHERE
products.name IN (
'TOASTMASTER 3000',
'TOASTMASTER 3000Rev01',
'A3452 Ultra Microwave Oven',
)
),
warehouseStock AS
(
--only works with one inventory
SELECT * FROM STOCK
WHERE stock.warehouse='WH-1'
)
-- left join to ensure all item categories are included
SELECT COALESCE(warehouseStock.amount,0) as amount,* FROM selProducts
LEFT JOIN warehouseStock ON selIProducts.itemId=warehouseStock.item
It tried replacing WHERE stock.warehouse='WH-1' with WHERE stock.warehouse IN ('WH-1','WH-2') but that doesn't work since the desired product types are only joined once, instead of once per warehouse.
The final result should look like this:
Warehouse productName amount wattage slots category
WH-1 TOASTMASTER 3000 0 null 2 toasters
WH-1 TOASTMASTER 3000Rev01 1 null 3 toasters
WH-1 A3452 Ultra Microwave Oven 1 3000 null microwave
WH-2 TOASTMASTER 3000 2 null 2 toasters
WH-2 TOASTMASTER 3000Rev01 0 null 3 toasters
WH-2 A3452 Ultra Microwave Oven 0 3000 null microwave
I don't know how I am I should get postgres to return a null when there isn't a stock in a given warehouse.
Does anybody have any ideas?

Answer 1

You seem to want all products and all warehouses. That suggests a cross join to generate the rows:
SELECT v.warehouse, p.productname,
COALESCE(s.amount, 0) as amount
FROM selProducts p CROSS JOIN
(VALUES ('WH-1'), ('WH-2')) v(warehouse) LEFT JOIN
stock s
ON p.itemId = s.item AND v.warehouse = s.warehouse;
You might have another source for the warehouses, if you don't want to list them explicitly.

Answer 2

Add a table of warehouses wanted.
WITH selIProducts AS(
SELECT
--Get the products category by checking if the table is part of the query
(CASE
WHEN toasters IS NOT NULL THEN 'toasters'
WHEN microwaves IS NOT NULL THEN 'microwaves'
ELSE 'ERROR'
END) as category,
products.name as productName,
*
FROM products
--I need a full join to include everything
FULL JOIN toasters ON toasters.name=products.name
FULL JOIN microwaves ON microwaves.name=products.name
WHERE
products.name IN (
'TOASTMASTER 3000',
'TOASTMASTER 3000Rev01',
'A3452 Ultra Microwave Oven',
)
),
warehousesWanted AS
(
SELECT *
FROM Warehouse
WHERE name in ('WH-1', 'WH-2')
)
-- left join to ensure all item categories are included
SELECT COALESCE(warehouseStock.amount,0) as amount, *
FROM selIProducts sp
CROSS JOIN warehousesWanted ww
LEFT JOIN Stock ON Stock.itemId = sp.itemId
and ww.Name = Stock.Warehouse;
You may need to correct ON clause as I'm not sure what are proper column names of your real tables.