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CS 157B Spring 2008

CS 157B Spring 2008. Prof. Sin Min Lee Presented by Li Ch’i Ooi. Manipulating Information with the Relational Algebra [Ch. 6.1]. Relation is a set of tuples and that each tuple in a relation has the same number and types of attributes. Relational algebra includes : Selection Operators

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CS 157B Spring 2008

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  1. CS 157B Spring 2008 Prof. Sin Min Lee Presented by Li Ch’iOoi

  2. Manipulating Information with the Relational Algebra [Ch. 6.1] • Relation is a set of tuples and that each tuple in a relation has the same number and types of attributes. Relational algebra includes : • Selection Operators • Projection Operators • Set Operators • Join and product Operations

  3. Selection Operators () • Reduce the number of tuples in a set by selecting those that satisfy some criteria. • Example : lastName = ‘Doe’ (Customer) [ Select from Customer where lastName = ‘Doe’ ] Customer

  4. Projection Operators () • Reduce the size of each tuple in a set by eliminating specific attributes. • Example : lastName, firstNAme (Customer) [ project customer onto (lastName, firstName) ] Customer

  5. Set Operators (  -) • Manipulate two similar sets of tuples by combining or comparing. • Example : Rental  PreviousRental Rental PreviousRental

  6. Set Operators (  -) ...con’t • The union of two relations is a relation that contains the set of each tuple that is in at least one of the input relations. • Partial result of the Rental  PreviousRental

  7. Set Operators (  -) ...con’t • The intersection of two relations is the set of all tuples that occur in both input relations. • The intersection of the relations Rental  PreviousRental in the previous example will return an empty set. • Another example would be the intersection between the video IDs of the two tables. •  videoId (Rental)   videoId (PrevioutsRental) = Videotapes that are currently rented as well as those that have been rented before. • The set of all videotapes that have been rented previously but are not currently rented is expressed as follows:  videoId (PreviousRental) -  videoId (Rental)

  8. Join and Product Operations () • Increase the size of each tuple by adding attributes • The Cartesian product produces a tuple of the new realtion for each combination of one tuple from the left operand and one tuple from the right operand. Example : Employee  TimeCard Employee TimeCard

  9. Join and Product Operations () ...con’t • The result of this operation has 30 tuples because there are 5 Employee and 6 TimeCard. • Partial result of Cartesian product Employee  TimeCard

  10. Join and Product Operations () ...con’t • A selection of those tuples where Employee.ssn equals TimeCard.ssn can be expressed by : Employee.ssn = TimeCard.ssn (Employee  TimeCard) • This type of product is called a join. The join operation puts together related objects from two relations. • A Natural Join however is defined so that the shared attribute appears only once in the output table. • Ref. textbook Table 6.6 [natural join] vs Table 6.7 [join]

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