Very often, you'll have the need to select either everything from a table or just a single record. This is the way to do both given a single query. When @EmployeeID is NULL, it will return all records from Employee, when a value is present, it will return only the value that is present.
-- Use one query to select a record given an identifier or all records when that ID is NULL
declare @EmployeeID int
set @EmployeeID = 42 -- NULL
where EmployeeID = coalesce(@EmployeeID, EmployeeID)