CS338 Lecture 5: Module 5
10 Feb 2019
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(dno = 4 and salary >2500) or (dno = 5 and salary >3000 (employee: different than sql select, correspondence, relational algebra: (r) Select operator properties: relational model is set-based (no duplicate tuples, thus selection cannot produce duplicates, c1 ( c2 (r)) = c2 ( c1 (r)) = c1 and c2 (r) Selectivity: fraction of tuples selected by a selection condition: Project operator properties: l(r) is defined only when l contained in attr(r, l2 ( l1 (r)) = l2 (r) L2 ( c (r)) = c ( l2) (r) as along as all attributes for c are in l: degree - number of attributes in projected attribute list. Working with long expressions: can be easier to write expressions piece by piece. Operators from set theory: merge elements of two sets in various ways, binary operators, relations must have the same type of types (union-compatible, union, r s, all tuples in r,s or both, duplicates removed.
