It seems that a lot of the INSERT and SELECT issues could be addressed via a stored function/procedure in a database such as PostgreSQL, MySQL's stored functions/procedures aren't there yet. You would be able to do the validation within the stored procedure, or even in a trigger, which would likely be even more efficient. I always find it odd when the solution is to use something such as Mongo DB, and continue to write more of your database logic in your app, rather than in your database, where it can run fastest without any unnecessary round-trips.
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It seems that a lot of the INSERT and SELECT issues could be addressed via a stored function/procedure in a database such as PostgreSQL, MySQL's stored functions/procedures aren't there yet. You would be able to do the validation within the stored procedure, or even in a trigger, which would likely be even more efficient. I always find it odd when the solution is to use something such as Mongo DB, and continue to write more of your database logic in your app, rather than in your database, where it can run fastest without any unnecessary round-trips.