Exists in sql server的执行

Question

As we know all are saying EXISTS is good over IN clause. I have question ever which not answered every time.

when we use Exists , is complete execution of inner query while it get first record or it will continue execution for all records??

Answer 1

both exists and in are the same. Any sane query plan optimizer will produce the same plan for them both.

-- http://sqlfiddle.com/#!6/97c87/1
select *
from t1
where name in (select name from t2)

This query will produce the plan:

-- http://sqlfiddle.com/#!6/97c87/2
select *
from t1
where exists (select name from t2 where t2.name=t1.name)

And this second query will produce the plan:

As you can see, both are identical.

Answer 2

[NOT] EXISTS operator gives best performance when the subquery ie driven query contains huge volume of data. The reason is that it follows the principle of 'At least found' in queries. It is set to TRUE, if at least one record is found in the subquery correlating with the main driving query, and stops further scanning of the table. Unlike other comparison operators like [NOT] IN, LIKE and others, which returns data for comparison, [NOT] EXISTS return BOOLEAN output.