SQL Using Multiply 3 Times with different ID's - sql

Question

I have tables called Products and ProductsDetails. I want to get something like the price of an order. So let's say I want 5 pairs of "Headphonesv1" ( Comparing with not ID but name, since name could change ), 2 packs of "GumOrbit" and 7 packs of "crisps". Pair of headphonesv1 costs 10$, gum 1$ and crisps 2$. So the answer that I should get is bill ID, Bill date, and TotalCost which is = 66. My question is how do I make multiple calculations? The code that I've been trying with one at least but I get syntax error:
SELECT Products.billID, Products.Date, (ProductsDetails.Price * 5 WHERE ProductsDetails.name LIKE 'Headphonesv1')
FROM Products INNER JOIN ProductsDetails ON Products.billdID = ProductsDetails.billID
Also have tried inserting SELECT inside () but then the values that I get are wrong and creating one more inner join inside doesn't seem promising

Answer 1

I think if you just want to see that total cost for multiple items you can use a aggregate and case expression to get the SUM.
SELECT Products.billID,
Products.Date,
SUM(CASE WHEN ProductsDetails.name LIKE 'Headphonesv1' THEN ProductsDetails.Price * 5
WHEN ProductsDetails.name LIKE 'GumOrbit' THEN ProductsDetails.Price * 2
WHEN ProductsDetails.name LIKE 'crisps' THEN ProductsDetails.Price * 7
END) TotalCost
FROM Products
INNER JOIN ProductsDetails ON Products.billdID = ProductsDetails.billID
GROUP BY Products.billID,
Products.Date
this seems very hard coded to serve much use though

Answer 2

Your previous code has a few synax erros. you are closing your parenthesis in the wrong location, and the FROM ... clause should be before the WHERE ... clause. Change your code to this:
SELECT Products.billID, Products.Date, (ProductsDetails.Price * 5)
FROM Products INNER JOIN ProductsDetails ON Products.billdID = ProductsDetails.billID
WHERE ProductsDetails.name LIKE 'Headphonesv1'
and let me know if it works now.
Edited:
The OP requested how to obtain that information for more than one Product. In that case you could switch the statement to:
SELECT SUM(Cost)
FROM (
SELECT Products.billID, Products.Date,
(ProductsDetails.Price *
(CASE WHEN ProductsDetails.name='Headphonesv1' THEN 5 ELSE
CASE WHEN ProductsDetails.name='GumOrbit' THEN 7 ELSE 0 END
END)
) AS Cost
FROM Products INNER JOIN ProductsDetails ON Products.billdID = ProductsDetails.billID
WHERE (ProductsDetails.name = 'Headphonesv1') OR (ProductsDetails.name = 'GumOrbit')
)
And you can continue to add more products.

Answer 3

You probably have another two tables: cart(cartID int/*,more info*/) and cartItems(cartID int /*FK*/, item varchar(50),qty int) where you add something. Finally,
select sum(pd.price * ci.qty) tot
FROM Products p
INNER JOIN ProductsDetails pd ON p.billdID = pd.billID
inner join cartItems ci on ci.item = pd.name
inner join cart c on c.cartID = ci.cartID
where c.cartID = 123456