anonxen anonxen - 2 months ago 8
SQL Question

Exists / not exists: 'select 1' vs 'select field'

Which one of the two would perform better(I was recently accused of not being careful with my code because I used the later in Oracle):

Select *
from Tab1
Where (not) exists(Select 1 From Tab2 Where Tab1.id = Tab2.id)


Select *
from Tab1
Where (not) exists(Select Field1 From Tab2 Where Tab1.id = Tab2.id)


Or are they both same?

Please answer both from SQL Server perspective as well as Oracle perspective.

I have googled (mostly from sql-server side) and found that there is still a lot of debate over this although my present opinion/assumption is the optimiser in both the RDMBS are mature enough to understand that all that is required from the subquery is a Boolean value.

Answer

Yes, they are the same. exists checks if there is at least one row in the sub query. If so, it evaluates to true. The columns in the sub query don't matter in any way.

According to MSDN, exists:

Specifies a subquery to test for the existence of rows.

And Oracle:

An EXISTS condition tests for existence of rows in a subquery.

Maybe the MySQL documentation is even more explaining:

Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.