Custom Fields SQL FILTER

How does one go about using SQL FILTERS on custom fields. For instance, we can get custom sql filters to work with pre-loaded fields. But if we create a custom field for say if someone has a pool in their backyard, and then try to filter on that. It does not work. The only information in the column for said field is Y. There is nothing other than that...and N will show up as null in the DB. Please advise if this is even possible, and if so how would I go about getting this to work.