18 note that the inner query can refer to e but the

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Unformatted text preview: column names are qualified by their relation names. 18 Note that the inner query can refer to E, but the outer query cannot refer to D. EXISTS AND UNIQUE FUNCTIONS [NOT] EXISTS function • Check whether result of correlated nested query is empty or not • EXISTS equivalent to (SELECT COUNT(*) … ) <> 0 Customer custid name address phone Sale saleid date custid SELECT name, phone FROM Customer C WHERE NOT EXISTS ( SELECT * FROM Sale S WHERE C.custid = S.custid); • Note that columns selected in inner query are irrelevant. SQL function UNIQUE(Q) 19 • Returns TRUE if no duplicate tuples in result of query Q SUMMARY Complex SQL: • • • • • Self joins Aggregate functions Grouping Sorting Nested queries Relational algebra expressions can handle self joins and nested queries with no additional operators 21 • Grouping, aggregations, and sorting require additional operators...
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