22 - the names of those students who should be considered for a prize and so we select those candidates who got A in the exam Starting from the

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We might wish to modify Student Exam by hiding the number informa- tion to get a new relation Results . This can be done by the operation projec- tion , to yield the following result: name address grade . . . . . . . . . Brown, B 5 Lawn Rd. A Smith, J 9 Elm St. A Walker, S 4 Ash Gr. C . . . . . . . . . In our logical notation, we may write: Results (n,a,g) if and only if no. Student Name (n,a,no,g) In database notation, this would be written in the style project Student Exam over ( name , address , grade ) Notice that Results is obtained from Student and Exam by a sort of gen- eralised composition. In fact, composition of binary relations can be con- structed by a join followed by a projection. Another operation we might wish to do is to select a part of a relation table which is of interest. Suppose in our registry example we wish to have
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Unformatted text preview: the names of those students who should be considered for a prize, and so we select those candidates who got A in the exam. Starting from the relation Results , we obtain a relation A-Results : name address grade . . . . . . . . . Brown, B 5 Lawn Rd. A Smith, J 9 Elm St. A . . . . . . . . . In logical notation, we could write A-Results (n,a,g) if and only if Results (n,a,g) g = A In database notation we have select results where grade = A The relation A-Results gives us the names we want, but we could reduce further to get the relation PrizeCands with the single attribute: 23...
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This note was uploaded on 01/02/2010 for the course MATH Math2009 taught by Professor Koskesh during the Spring '09 term at SUNY Empire State.

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