A
generalized selection is a
unary operation written as \sigma_\varphi(R) where \varphi is a
propositional formula that consists of
atoms as allowed in the normal selection and, in addition, the logical operators ∧ (
and), ∨ (
or) and \lnot (
negation). This selection selects all those
tuples in for which \varphi holds. For an example, consider the following tables where the first table gives the relation and the second the result of \sigma_{\text{Age} \ge 30 \ \land \ \text{Weight} \le 60}(\text{Person}). | Formally the semantics of the generalized selection is defined as follows: : \sigma_\varphi(R) = \{ \ t : t \in R, \ \varphi(t) \ \} The result of the selection is only defined if the
attribute names that it mentions are in the
header of the relation that it operates upon. The generalized selection is expressible with other basic algebraic operations. A simulation of generalized selection using the fundamental operators is defined by the following rules: : \sigma_{\varphi \land \psi}(R) = \sigma_\varphi(R) \cap \sigma_\psi(R) : \sigma_{\varphi \lor \psi}(R) = \sigma_\varphi(R) \cup \sigma_\psi(R) : \sigma_{\lnot \varphi}(R) = R - \sigma_\varphi(R) ==Computer languages==