Consider this query: Retrieve the Ssns of employees who work on at least those projects on which the employee with Ssn=123456789 works. This may be stated as (FORALL x) (IF P THEN Q), where

■ x is a tuple variable that ranges over the PROJECT relation.

■ P ≡ employee with Ssn=123456789 works on project x.

■ Q ≡ employee e works on project x.

Express the query in tuple relational calculus, using the rules

■ (∀ x)(P(x)) ≡ NOT(∃x)(NOT(P(x))).

■ (IF P THEN Q) ≡ (NOT(P) OR Q).