An operational methodology, founded on predicate calculus, dealing with descripitive expressions that are equivalent to the operations of relational algebra. Codd’s reduction algorithm can convert from relational calculus to relational algebra. Two forms of the relational calculus exist: the tuple calculus and the domain calculus. Codd proposed the concept of a relational calculus (applied predicate calculus tailored to relational databases).

It is founded on a branch of mathematical logic called the predicate calculus. Relational calculus is a formal query language where we write one declarative expression to specify a retrieval request and hence there is no description of how to evaluate a query; a calculus expression specifies what is to be retrieved rather than how to retrieve it. Therefore, the relational calculus is considered to be a nonprocedural language. This differs from relational algebra, where we must write a sequence of operations to specify a retrieval request; hence it can be considered as a procedural way of stating a query. It is possible to nest algebra operations to form a single expression; however, a certain order among the operations is always explicitly specified in a relational algebra expression. This order also influences the strategy for evaluating the query.

It has been shown that any retrieval that can be specified in the relational algebra can also be specified in the relational calculus, and vice versa; in other words, the expressive power of the two languages is identical. This has led to the definition of the concept of a relationally complete language. A relational query language L is considered relationally complete if we can express in L any query that can be expressed in relational calculus. Relational completeness has become an important basis for comparing the expressive power of high-level query languages. However certain frequently required queries in database applications cannot be expressed in relational algebra or calculus. Most relational query languages are relationally complete but have more expressive power than relational algebra or relational calculus because of additional operations such as aggregate functions, grouping, and ordering.