Could someone explain how Count works in this SQL query?

Question

I'm making a table in SSRS that contains customer names in column 1 and their corresponding number of orders in column 2. This query works for what I'm trying to accomplish, but I don't know exactly how the Count function knows what the heck I want it to count and what table I'm wanting it to count from. Could someone please explain this to me so I can better understand in the future? Thanks a ton.

SELECT Customers.name
    ,Count(1) AS OrderCount
FROM Customers
INNER JOIN Orders
    ON Customers.id = Orders.customer_id
GROUP BY Customers.name

Answer 1

I don't know exactly how the Count function knows what the heck I want it to count

There is only one thing that COUNT is able to count - it can count rows in which an expression evaluates to a non-null value. If you use COUNT(1) in a regular query, you would get 1 on each row. With GROUP BY, however, the COUNT will return the number of rows in the specific group. In your case, that would be the number of rows with the same Customers.name, because that is what you use for GROUP BY.

As far as passing 1 to COUNT goes, a more common practice these days is to pass an asterisk, i.e. to write COUNT(*), because in most RDBMS engines there is no performance penalty for that.