Revision section 3

Question 1

What do you need to do for:

Write the relational algebra expression for the following query:
SELECT E.NAME, P.NAME
FROM EMPLOYEE E, PROJECT P
WHERE P.PID = E.PID AND E.SALARY > 50000

Answer 1

Write the canonical expression

project(select(join))

It doesn’t ask for the most optimal.

Question 2

When creating an optimisation tree, if there are only 2 relations which one goes on the LHS?

Answer 2

The PK.

Question 3

What is the IN operator for?

Answer 3

It is short hand for OR

ie., ID in (1,2) is ID in 1 or 2