[PommeDeTerre]

This sounds like a situation where it'd just be better to use separate, yet similar, queries, with each handling a particular case or set of conditions.

Views, stored procedures and functions can be used to help isolate duplication, parameterize the queries, or otherwise hide the SQL.

Code like you've posted is the result of taking DRY too far, to the point where avoiding a small amount of repetition ends up bringing in far more complexity and problems than the repetition might cause.