GATE CS 2002 Q1.20 — Tuple Calculus and SQL

According to Codd's Theorem, the formal relational query languages have the following relationship regarding their expressive power:

• Relational Algebra is a procedural language, which always yields finite results for finite input relations.
• Relational Calculus (tuple or domain) is a declarative language. In its unrestricted form, it can express "unsafe" queries that produce infinite relations (for example, $\{ t \mid \neg (t \in R) \}$ when the domain is infinite).
• To prevent infinite results, we restrict the calculus to safe relational calculus.

Codd's Theorem proves that Relational Algebra is equivalent in expressive power to safe relational calculus (both safe tuple relational calculus and safe domain relational calculus). A language that has this level of expressive power is said to be relationally complete.

Thus, the correct option is C.