SQL减去不同表上的两列

Question

I have two tables, Customer, and Invoices,I have to list all the customers with their remaining credit. I'm trying to subtract the sum of the Invoice Amount column from the CreditLimit column to give me the remaining credit?

This is my query so far

DECLARE @CreditRemaining INT

SELECT 
   @CreditRemaining = (c.CreditLimit - SUM(i.Amount))
FROM 
     Customer c 
INNER JOIN 
     Invoices i on
     c.ID = i.customerId

Answer 1

Use a derived table for the invoice amount SUM() , then JOIN back to Customer:

DECLARE @CreditRemaining INT

SELECT @CreditRemaining = (c.CreditLimit - TotalSpent)
FROM Customer c
INNER JOIN (SELECT SUM(Amount) TotalSpent, CustomerID
            FROM Invoices
            GROUP BY CustomerID) i ON i.CustomerID = c.ID

As others mentioned, this is assuming you are limiting your selection to one customer.

For all customers, just use a select:

SELECT C.Name, (c.CreditLimit - TotalSpent) CreditRemaining
FROM Customer c
INNER JOIN (SELECT SUM(Amount) TotalSpent, CustomerID
            FROM Invoices
            GROUP BY CustomerID) i ON i.CustomerID = c.ID
GROUP BY C.Name