Explain Domain calculus

Domain calculus is one of the types of the relational calculus. The formal specification of the domain calculus was planned after the development of the QBE system. Domain calculus has the variables range in excess of single value from domains of attributes. To create a relation degree n  for a query result, we should have  n domain variables - one for each attribute. An expression of the domain calculus is of the form {X | F(X)}

In which F is a formula on X and X denotes a set of domain variables. The expression characterizes X like F(X) is true.