Answer Posted / subathra

IN: Inner query executes first and drives the outer query.
EXISTS: Outer query executes first and compares tge rows
retrived with the inner query.
Consider table tab1 has 1000 rows and table tab2 has 1000
rows.
IN: select t1.empid from tab1 t1 where t1.code in (select
t2.code from tab2 t2) -- All rows in t1 will read with t2
and the effect is 1000 X 1000 rows.
EXISTS: select t1.empid from tab1 t1 where exists (select 1
from tab2 t2 where t1.code=t2.code) -- Max of 1 row will be
read for each row of t1 and thus reduces the processing
overhead.
Thumb rule:
1) If the majority of the filtering are in the sub query
then use IN.
1) If the majority of the filtering are in the outer query
then use EXISTS.

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