Question

When we do a query such as this

SELECT COUNT(*)
FROM A, B, C
WHERE A.type = B.type
AND B.cost = C.cost

What can we say about the result table? What will the size of the result table be?
If A has 5 rows, B has 8 rows, and C has 7 rows

EDIT: Can I please get some response instead of mark down? I am total new to SQL and need help.

EDIT2: What I am trying to do – Understand how WHERE and AND clause works & why the result table has 8 rows

How you came to this particular question – I was reading my textbook and doing the questions in it

What I’ve tried to solve it – I tried Google and reading various of posts but most of them just shows HOW to do it instead of explaining what’s actually happpening

Answer 1

The count will be the number of common values . E.g. Consider this small example

DECLARE @A Table(Val1 Int)
Insert Into @A Values(1),(2),(3),(4),(5)


DECLARE @B Table(Val2 Int)
Insert Into @B Values(1),(2),(3),(4),(5),(6)


DECLARE @C Table(Val3 Int)
Insert Into @C Values(1),(2),(3),(4),(5),(6),(7)


SELECT COUNT(*)
FROM @A a, @B b, @C c
WHERE a.Val1 = b.Val2
AND b.Val2 = c.Val3

The result is 5. Because a.Val1 = b.Val2 has 5 common elements viz 1,2 3,4,5. Now b.Val2 = c.Val3 has 6 common elements viz 1,2,3,4,5,6.

Then these two result sets will act upon each other and will try to get the intersecting values i.e. common values which is again 1,2,3,4,5. Hence is the count as 5