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Unformatted text preview: Relational algebra operator overview Selection The operator P applied to a relation returns all tuples for which the predicate (or condition) P is true. E.g. Length > 150 Projection The operator A1,A2,...,Ak applied to a relation returns a relation with only the attributes A1, A2, ..., Ak. E.g. Title, Year (Movie) Union If we have two relations R and S, then R S is the relation formed by combining all the tuples of R and S. The relations must be union-compatible: that is, They have the same schema The matching attributes must have the same domain Intersection If we have two relations R and S that are union-compatible, R S is the relation that contains all tuples in that are in both R and S. Difference If we have two relations R and S that are union-compatible, R -- S is the relation that contains all tuples in R that are not in S. Cartestian Product If we have two relations R and S, the Cartesian product, R S, is the relation formed by combining every tuple from R with every tuple from S. Conditional Join If we have two relations R and S, the conditional join of R and S, R C S, is the relation obtained by combining every tuple from R with every tuple from S that satisfy the condition C. E.g. Movies Length > 150 && Movies.Year == StarsIn.Year StarsIn Natural Join A natural join R S of relations R and S is one in which we combine every tuple from R with every tuple from S that agree on whatever attributes R and S have in common. E.g. Movies StarsIn Left Outer Join A left outer join R S of relations R and S is a natural join that includes additional tuples in R that did not have matching values in the common attributes of S. Missing values from S are set to null. E.g Movies StarsIn ...
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This note was uploaded on 04/01/2008 for the course INFO 2300 taught by Professor Williamson during the Spring '08 term at Cornell University (Engineering School).
- Spring '08