Hi Paul & Jon,
SOQL is salesforce object query language and mainly for salesforce and it is leveraged via external calls and auto synchs combination, depending upon the task. If you have utilized this method, then you can use the advance filter option to query against your database, if not and you are using any other CRM systems with SFTP, you need look into other methods on how you build an interface or query to pull relevant contacts or accounts associations with combinations of custom objects. My recommendation would be look into either developing your cloud app, or in house interface using API. An example of UI can be seen on Beta app being developed by Fred Sakr http://topliners.eloqua.com/community/do_it/blog/2013/05/08/using-the-content-embed-widget-beta
Overall it requires little more investigation and broader discussion to come to agreement on right approach.
Hope this helps.
Thank You
Amit Pandya