Query Processing and Optimization

Unnesting Nested Queries, cont.
• Watch out for duplicates! Consider a query and its
rewritten counterpart.
SELECT AVG(salary)
FROM emp
WHERE manager IN (SELECT manager FROM techdept)
• Unnested version, with problems: (what’s the problem?)
SELECT AVG(salary)
FROM emp, techdept
WHERE emp.manager = techdept.manager
• This query may yield wrong results! A solution:
SELECT DISTINCT (manager) INTO temp
FROM techdept
SELECT AVG(salary)
FROM emp, temp
WHERE emp. manager = temp. manager
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