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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

CS 157B Spring 2008

Prof. Sin Min Lee

Presented by Li Ch’iOoi


Manipulating information with the relational algebra ch 6 1
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


Selection operators
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


Projection operators
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


Set operators
Set Operators (  -)

  • Manipulate two similar sets of tuples by combining or comparing.

  • Example : Rental  PreviousRental

    Rental PreviousRental


Set operators con t
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


Set operators con t1
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)


Join and product operations
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


Join and product operations con t
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


Join and product operations con t1
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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