Question

Here are two example of query for the same purpose
(in this example, I want to fine salesman from same city where Patriks lives.)

select * 
from salesman 
where salesman.city = 
     ( select city 
       from salesman 
       where salesman_name='Patriks'
     );

and

select s1.* 
from salesman s1,salesman s2 
where s1.city=s2.city 
  and s2.salesman_name='Patriks';

Which is better or efficient and why?
(I know this is small example, but I want to figure out, which will be good in complex situation, and for big database.)

Answer 1

As a general rule of thumb:

If you use a subquery, you force Oracle to use a certain execution path (ie it must execute the sub-query before it can execute the outer query)

If you use a join, Oracle is free to pick whichever it considers to be the most efficient path.

I would always go for the join over the subquery therefore. YMMV.