Unsafe Queries, Expressive Power
It is possible to write syntactically correct calculus queries that have an infinite number of answers! Such queries are called unsafe.
It is known that every query that can be expressed in relational algebra can be expressed as a safe query in DRC / TRC; the converse is also true.
Relational Completeness: Query language (e.g., SQL) can express every query that is expressible in relational algebra/calculus.