Q$4.$ Aggregate functions can be applied to a particular column $($attribute$),$ that is$,$ a collection of values.

$($a$)(4\%)$ Explain how the NULL values are handled when the SUM function is applied to the attribute Hours in the WORKS$\_$ON table. If the collection becomes empty because all values are NULL, what will the SUM function return?

$($b$)(2\%)$ If the collection becomes empty because all values are NULL, what will the COUNT function return?

$($c$)(2\%)$ Explain how will the COUNT${(}^{**})$ handle the tuple $($of the query result$)$ that contain NULL values of some attributes in the tuple?