Please review the explanation part for this answer
Explanation/Reference:
Explanation:
WHERE header.SalesPersonID IS NULL
On line 8 add: IS NULL to get:
CREATE FUNCTION dbo.OrdersWithNoSalesPerson(@ProductID INT)
RETURNS
AS
header.SalesOrderID
FROM Sales.SalesOrderDetail detail
INNER JOIN Sales.SalesOrderHeader header
on header.SalesOrderID = detail.SalesOrderID
WHERE header.SalesPersonID IS NULL
AND detail.ProductID = @ProductID;
GO
Note: IS NULL determines whether a specified expression is NULL. If the value of expression is NULL, IS NULL returns TRUE; otherwise, it returns FALSE.
To determine whether an expression is NULL, use IS NULL or IS NOT NULL instead of comparison operators (such as = or !=). Comparison operators return UNKNOWN when either or both arguments are NULL.
References:https://msdn.microsoft.com/en-us/library/ms188795.aspx