Question

I used to write my EXISTS checks like this:

IF EXISTS (SELECT * FROM TABLE WHERE Columns=@Filters)
BEGIN
   UPDATE TABLE SET ColumnsX=ValuesX WHERE Where Columns=@Filters
END

One of the DBA’s in a previous life told me that when I do an EXISTS clause, use SELECT 1 instead of SELECT *

IF EXISTS (SELECT 1 FROM TABLE WHERE Columns=@Filters)
BEGIN
   UPDATE TABLE SET ColumnsX=ValuesX WHERE Columns=@Filters
END

Does this really make a difference?

Answer 1

No, SQL Server is smart and knows it is being used for an EXISTS, and returns NO DATA to the system.

Quoth Microsoft:
http://technet.microsoft.com/en-us/library/ms189259.aspx?ppud=4

The select list of a subquery
introduced by EXISTS almost always
consists of an asterisk (*). There is
no reason to list column names because
you are just testing whether rows that
meet the conditions specified in the
subquery exist.

To check yourself, try running the following:

SELECT whatever
  FROM yourtable
 WHERE EXISTS( SELECT 1/0
                 FROM someothertable 
                WHERE a_valid_clause )

If it was actually doing something with the SELECT list, it would throw a div by zero error. It doesn’t.

EDIT: Note, the SQL Standard actually talks about this.

ANSI SQL 1992 Standard, pg 191 http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt

3) Case:
a) If the <select list> “*” is simply contained in a <subquery> that
is immediately contained in an <exists predicate>, then the <select list> is
equivalent to a <value expression>
that is an arbitrary <literal>.