What are the benefits of using Exists ?

Many times you're required to write query to determine if a record exists. Typically you use this to determine whether to insert or update a records. Using the EXISTS keyword is a great way to accomplish this.
Here's a simple example from the pubs database using EXISTS:

if EXISTS (select *
 from authors
 where au_id = '172-32-1176')
  Print 'Record exits - Update'
ELSE
  Print 'Record doesn''t exist - Insert'

The EXISTS function takes one parameter which is a SQL statement. If any records exist that match the criteria it returns true, otherwise it returns false. This gives you a clean, efficient way to write a stored procedure that does either an insert or update.
The other benefit of EXISTS is that once it finds a single record that matches it stops processing. This doesn't have a huge impact if you're checking on a primary key. It does have a big impact if you're checking for existance based on another field. Consider the following two queries:

if exists (select *
 from authors
 where state = 'ca')
  Print 'Record exits'
ELSE
  Print 'Record doesn''t exist'

if (select count(*)
 from authors
 where state = '172-32-1176') > 0
  Print 'Record exits'
ELSE
  Print 'Record doesn''t exist'

In the pubs database there are only 23 records in the authors table.  
Even with that small number of records, the IF EXISTS version runs 4 
times faster than selecting a count.  This is because it stops as soon 
as it finds a single record that matches the criteria.  The second 
statement must process all the rows that match.