Selection

Definition

Given a relation instance R over set of attributes U and a condition F, the selection σ _F (R) returns a new relation over U consisting of the set of tuples of R which satisfy F. The condition F is an atom of the form A = B or A = c, where A and B are attributes in U, and c is a constant value.

The generalized selection allows more complex conditions: F can be an arbitrary boolean combination of atoms of the form A = B or A≠B or A = c or A≠c. Moreover, if a total order is defined on the domain of attributes, more general comparison atoms of the form A α B or A α c are allowed, where α ranges over {=, ≠, <, >, ≤, ≥}.

Key Points

The selection is one of the basic operators of the relational algebra. It operates by “selecting” rows of the input relation. A tuple t over U satisfies the condition A = B if the values of attributes A and B in t are equal. Similarly t satisfies the condition A = c if the value of attribute A in t is c. Satisfaction of generalized selection atoms is...