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21 - join to get a new relation which we call Student Exam...

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attribute is a set (or domain ) from which it takes its values. It is clear that these database relations are just the same as the n -ary relations we have been studying. In our example, we may write Student name set × address set × number set using an obvious notation for the sets associated with each attribute. Suppose that the registry database has another relation, called Exam , which records the results for students taking the compilers exam. It has attributes number and grade . A table for Exam might look like number grade . . . . . . 105 A 156 A 189 C . . . . . . Notice that the relations Student and Exam share an attribute, namely num- ber . We can combine the two relations using an operation called
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Unformatted text preview: join to get a new relation, which we call Student Exam , which merges the two relations on their common attribute: name address number grade . . . . . . . . . . . . Brown, B 5 Lawn Rd. 105 A Smith, J 9 Elm St. 156 A Walker, S 4 Ash Gr. 189 C . . . . . . . . . . . . Notice that candidate 167 did not sit the exam, and so therefore does not appear in the join. We can deFne this join operation quite easily using our logical notation: Student Exam (n,a,no,g) ⇔ Student (n,a,no) ∧ Exam (no,g) In the language of database theory, it is usually given a more readable form, such as join Student and Exam over number 22...
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