I am reinventing this particular wheel right now. I am working on a "mart" type application; the idea is basically to have common cases (and not so common, but complicated ones) wrapped into their own classes; each provides a sql() method, which are collated together to create the final query.

I use Class::DBI to store these components in a database: as Fields, Filters, Joins and Groups (purely an organizational thing). You can get some cute optimizations with this, for example if a field is selected from another table it usually is inserted as a subselected, but if a filter is added on that same table, a join (inner) is automatically included and the field uses that as well.

The real advantage of this for me is that it provides methods to quickly construct html forms for these queries (coupled with Template::Toolkit a Filter "knows" how to draw itself), after which I can just give the parsed CGI.pm query object to the SQL thing and get back the complete SQL statement. Makes working with sessions trivial as well.

I am not sure how many people would find this sort of thing useful, but in this case it's been working pretty well so far.