Answer Posted / lova raju allumalla

when performing joins on two or more tables, we normally
write a query making use relational operators such as =
etc. which means each and every row of every table is
compared with each other,which is a unwanted situation when
looked in the perspective performance issue. where as
making use of corelated query one can minimise the tedoius
checks as described above. consider the example below with
a normal join
select * from emp e,dept d where e.deptno=d.deptno;

now the above same can be performed by using the co related
query
select * from emp where deptno in (select deptno from dept);

u can now make up the difference, in the first approach in
emp table, each row deptno value compared for each row
deptno value of dept table.where as in second approach the
(select deptno from dept) is executed only once for all
checks of the outer query select * from emp where deptno

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