How to find the distinct values of a particular field in a SObject using SOQL Query?
Its possible by using the GROUP BY Clause in an SOQL Query.
For example:
Select Name from Account GROUP BY Name – This will give all the distinct values for the Account Name field from Account Object.
For More Details: http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_calls_soql_select_groupby.htm