Define Atom of domain relational calculus
An atom has the outline < x1, x2, ... , xn > -> r , in which r stands for a relation on n attributes and x1, x2, ... , xn are the domain variables or domain constraints.
x Θ y , in which x and y are domain variables and Θ is the comparison operator (<, >, <, >, =, ≠). It is needed that x and y have domains which can be compared by Θ.
x Θ c, in which x is a domain variable, Θ is a comparison operator, and c is a constraint in the domain of attributes for which x is a domain variable.