Editing Relational algebra (section)

==== Breaking up selections with complex conditions ====

A selection whose condition is a [[Logical conjunction|conjunction]] of simpler conditions is equivalent to a sequence of selections with those same individual conditions, and selection whose condition is a [[Logical disjunction|disjunction]] is equivalent to a union of selections.  These identities can be used to merge selections so that fewer selections need to be evaluated, or to split them so that the component selections may be moved or optimized separately.

#<math>\sigma_{A \land B}(R)=\sigma_{A}(\sigma_{B}(R))=\sigma_{B}(\sigma_{A}(R))</math>
#<math>\sigma_{A \lor B}(R)=\sigma_{A}(R)\cup\sigma_{B}(R)</math>