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Mastering Oracle SQL
book

Mastering Oracle SQL

by Sanjay Mishra, Alan Beaulieu
April 2002
Intermediate to advanced content levelIntermediate to advanced
336 pages
9h 58m
English
O'Reilly Media, Inc.
Content preview from Mastering Oracle SQL

Joins and Subqueries

Joins can sometimes be used to good advantage in reformulating SELECT statements that would otherwise contain subqueries. Consider the problem of obtaining a list of suppliers of parts for which your inventory has dropped below ten units. You might begin by writing a query such as the following:

SELECT supplier_id, name
FROM supplier s
WHERE EXISTS (SELECT *
              FROM part p
              WHERE p.inventory_qty < 10
                AND p.supplier_id = s.supplier_id);

The subquery in this SELECT statement is a correlated subquery, which means that it will be executed once for each row in the supplier table. Assuming that you have no indexes on the INVENTORY_QTY and SUPPLIER_ID columns of the PART table, this query could result in multiple, full-table scans of the PART table. It’s possible to restate the query using a join, for example:

SELECT s.supplier_id, s.name
FROM supplier s, part p
WHERE p.supplier_id = s.supplier_id
  AND p.inventory_qty < 10;

Whether the join version or the subquery version of a query is more efficient depends on the specific situation. It may be worth your while to test both approaches to see which has a lower cost.

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Publisher Resources

ISBN: 0596001290Supplemental ContentCatalog PageErrata