# relational algebra:Set Operations on Relations

May 29th, 2015
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For the set operations on relations, both operands must have the same scheme, and the result has that same scheme. •R1 U R2 (union) is the relation containing all tuples that appear in R1, R2, or both. •R1 n R2 (intersection) is the relation

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Relational Algebrais a set of relations closed under operators. Operators operate on one or more relations to yield a relation. Relational algebra is a part of computer science.Algebra > a formal structure consisting of sets and operations on those sets. Relational algebra is a formal system for manipulating relations. Operands of this algebra are relations. Operations of this algebra include the usual set operations (since relations are sets of tuples), and special operations defined for relations selection projection join Set Operations on RelationsFor the set operations on relations, both operands must have the same scheme, and the result has that same scheme. R1 U R2 (union) is the relation containing all tuples that appear in R1, R2, or both. R1 n R2 (intersection) is the relation containing all tuples that appear in both R1 and R2. R1 - R2 (set difference) is the relation containing all tuples of R1 that do not appear in R2. SelectionSelects tuples from a relation whose attributes meet the selection criteria, which is normally expressed as a predicate. R2 = select(R1,P) That is, from R1 we create a new relation R2 containing those tuples from R1 that satisfy (make true) the predicate P. A predicate is a boolean expression whose operators are the logical connectives (and, or, not) and arithmetic comparisons (LT, LE, GT, GE, EQ, NE), and whose operands are either domain names or domain constants. select(Workstation,Room=633) =

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