EXISTS vs. COUNT(*)

Often times I see the following query used to determine if records exist:

IF ((SELECT COUNT(*) FROM Business) > 0)

BEGIN

–Do some stuff

END

My first guess would be that SQL Server would actually perform a count of all of the records in the Business table in this instance.  It turns out that it doesn’t.  It seems that this query gets turned into:

IF EXISTS (SELECT * FROM Business)

BEGIN

–Do some stuff

END

In this case, the execution plans are identical and STATISTICS IO shows the same exact I/O.

What is interesting is that if you change the COUNT(*) query to check against anything other than 0, a full counting of the records does occur.  For example:

IF ((SELECT COUNT(*) FROM Business) > 1)

BEGIN

–Do some stuff

END

This query does execute the full SELECT COUNT(*) against the table.  You might think that it would stop after reaching the desired value (in this case 1), but apparently it does not.

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SQL Server Expert, Steve Abraham, holds 8 Microsoft certifications and specializes in SQL Server and .Net Framework architecture, high availability, capacity planning, development, and performance tuning.