I have two doubts:
1) A natural join B
What will be the result when B don't have any tuple in it. Like in B their is relational algebra expression which is selecting Cars of green color but their is no green color car in table. What will be the result in this case.
2) A / B
In above divison if B is a relational algebra expression returning nothing ( like the B in case 1). What will be the result ?