CORRELATED SUB QUERY:
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1.if we have realtime requirement than only should use the correlated subquery otherwise
not use why because it will decrease the query level
performance
and data level performance.
CORRELATED SUB QUERY
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OUTER----------------SUBQUERY
2.the subquery which is depending on outer query result is called a correlated subquery.
3.Generally the correlated sub query usefull/fetch
reverse buisness process information.
Means suppose
SAMSUNG company manufacturing some
mobiles ,after one year if we want to see information about the products that is there any mobile model which is not sold to
atleast one customer?.
4.In correlated subquery between outer query and sub
query we use two operators those are EXISTS, NOT EXISTS.
5. what is EXISTS?
EXISTS operator
always observing in subquery o/p that is if atleast one o/p value is generated than EXISTS operator returns TRUE. If EXISTS
operator returns TRUE than outer query
will execute result,if EXISTS operator returns FALSE than outer query will not
execute result.
6. INNER
PROCESSING OF THE CORRELATED SUBQUERY
CORRELATED
SUBQUERY
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1 O/P
3
O/P 2
Ø The
sub suery which is depending on outer
query that is the reason outer query will execute first once execute the outer
query(1) it is generate some o/p later that
o/p will passed to the subquery based on that o/p the subquery executed one
time(2) it will generate o/p and passed to the outer query here again executed
outer query(3) based on the sub query o/p finally answer will executed.
Ø According
to the above process outer query executing two times.that is the reason until requirement we do not create the correlated sub queries.
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