>I am developing an application that includes financial accounting.
>Should I store data for multiple companies in a single Database?
>Is it possible to apply a filter on a single field say company_id in all data tables globally so that I may operate a single company at a time. I want to avoid including this condition in each & every where clause throughout the application. I also need to generate some combined reports, where data from multiple companies would be required.

Having them all in a single database would complicate things IMHO. Doing cross database joins is easy. You could also create shared stored procedures, functions etc. However don't take that as I'm saying do it absolutely separate. If you trust your coding then you might as well do it in a single database, use view/table partioning etc.
Cetin