Can someone please explain how the answers for the fisrt 3 (and perhaps the last

Question

Can someone please explain how the answers for the fisrt 3 (and perhaps the last 2..) were derived? I am familiar with relational algebra, but these are a bit tricky

Each row of the following table shows two queries. In the blank third column of the table write "YES" if the two queries are equivalent, and "NO" if they are not equivalent. Two queries are equivalent if they always return exactly the same answer on all databases All queries refer to a schema containing two relations ·R(A, B) where A is the primary key and B is a candidate key ·S(A, B) where A is the primary key (and only key) You may assume that the relations do not contain null values, but do not make any other assumptions about the relations Query 1 Equivalent? (YES/NO) NO YES Query 2 RN S SELECT DISTINCT R.A SELECT R.A FROM R, S WHERE R.B#8 . B SELECT R.A, COUNT(SELECT R.A, COUNT*) FROM R WHERE R. A>3 GROUP BY R.A NO FROM R, S WHERE R,B=S . B YES FROM R GROUP BY R.A HAVING R. A>3

Explanation / Answer

Solution:

1)

They are not equal because Query 1 will result in the row A after the natural join is done, based on the common row between them.

and Query 2 will result in only based on A after it is extracted.

2)

Here, both the queries are same because both are performing Natural join eventually.

3)

The result of query 1 is the natural join between R and S, on row B

and in query 2 the selection A where RcapS will give the A and B where A is common but the natural join will result in all the rows where B is common.

4)

In query 1 DISTINCT is used to extract unique rows from the table, wherease in query 2 all the replicated entries will also be there.

5)

The order between Where and Group by doesn't matter so query 1 and query 2, both are the same.

I hope this helps if you find any problem. Please comment below. Don't forget to give a thumbs up if you liked it. :)

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