11. Functions: scalar, inline TVF, multi-statement TVF

-- Scalar function
CREATE OR ALTER FUNCTION dbo.fn_TaxAmount (@Amount DECIMAL(12,2)) RETURNS DECIMAL(12,2)
AS BEGIN RETURN @Amount * 0.18; END;

-- Inline table-valued function (best performing — expanded like a view)
CREATE OR ALTER FUNCTION dbo.fn_OrdersByCustomer (@CustomerId INT) RETURNS TABLE
AS RETURN (SELECT OrderId, OrderDate, Total FROM dbo.Orders WHERE CustomerId = @CustomerId);

SELECT * FROM dbo.fn_OrdersByCustomer(42);
SELECT c.Name, o.* FROM Customers c CROSS APPLY dbo.fn_OrdersByCustomer(c.CustomerId) o;