# relAlg - Relational Algebra Goal Some operators to...

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Himanshu Gupta CSE 532 – RelAlg – 1 Relational Algebra Goal: Some operators to “manipulate” tables. Relational operators can be looked as analogous to the arithmetic operators which are used to manipulate numbers.

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Himanshu Gupta CSE 532 – RelAlg – 2 Core” Relational Algebra A small set of operators that allow us to manipulate relations in limited but useful ways. The operators are: 1. Union, intersection, and difference: the usual set operators. But the relation schemas must be the same. 2. Selection : Picking certain rows from a relation. 3. Projection : Picking certain columns. 4. Products and joins : Composing relations in useful ways. 5. Renaming of relations and their attributes.
Himanshu Gupta CSE 532 – RelAlg – 3 Selection R 1 = σ C ( R 2 ) where C is a condition involving the attributes of relation R 2 . Example Relation Sells : JoeMenu = σ bar =  Joe’s ( Sells ) bar beer price Joe's Bud 2.50 Joe's Miller 2.75 Sue's Bud 2.50 Sue's Coors 3.00 bar beer price Joe's Bud 2.50 Joe's Miller 2.75

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Himanshu Gupta CSE 532 – RelAlg – 4 Projection R 1 = π L ( R 2 ) where L is a list of attributes of R 2 . Example π beer,price ( Sells ) Notice elimination of duplicate tuples. beer price Bud 2.50 Miller 2.75 Coors 3.00
Himanshu Gupta CSE 532 – RelAlg – 5 Cartesian Product R = R 1 × R 2 pairs each tuple t 1 of R 1 with each tuple t 2 of R 2 and puts in R a tuple t 1 t 2 .

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Himanshu Gupta CSE 532 – RelAlg – 6 Theta-Join R = R 1 C R 2 is equivalent to R = σ C ( R 1 × R 2 ).
Himanshu Gupta CSE 532 – RelAlg – 7 Example Sells =                                   Bars = BarInfo = Sells Sells.Bar=Bars.Name Bars bar beer price Joe's Bud 2.50 Joe's Miller 2.75 Sue's Bud 2.50 Sue's Coors 3.00 name addr Joe's Maple St.

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