Editing Relational algebra (section)

== Selection ==
{{Main|Selection (relational algebra)}}
A '''generalized selection''' (&sigma;) is a [[unary operation]] written as <math>\sigma_\varphi(R)</math> where {{φ}} is a [[propositional formula]] that consists of [[atomic formula|atom]]s as allowed in the [[selection (relational algebra)|normal selection]] and the logical operators {{and}} ([[logical conjunction|and]]), {{or-}} ([[logical disjunction|or]]) and {{Not}} ([[negation]]). This selection selects all those [[tuple]]s in ''R'' for which {{φ}} holds.

To obtain a listing of all friends or business associates in an address book, the selection might be written as <math>\sigma_{\text{isFriend = true} \,\lor\, \text{isBusinessContact = true}}( \text{addressBook} )</math>. The result would be a relation containing every attribute of every unique record where {{math|size=90%|isFriend}} is true or where {{math|size=90%|isBusinessContact}} is true.