What is the difference between correlated sub queries and
inner queries?
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Answer Posted / vinay sir(datawarehousing tech
Guys,
First of all the question is not proper,it is supposed to
be "the difference between correlated sub queries and
simpe sub queries".
Corelated and Simple, both are Subqueries(having inner and
outer queries) but the way they execute is different.
Simple Subquery:Inner query executed first and then outer
query. Inner query executed only one time.
Corelated Sub query:Outer query is executed first and then
inner query.Outer query is executed many times.
Please find a common example where you can see the
execution difference.
Simple Subquery: select max(sal) from emp where sal not in
(select max(sal) from emp)
Corelated: Select a.sal from emp e where 1=(select count
(distinct(b.sal)) from emp b where a.sal<b.sal)
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