A nested database query can be optimized by effectively representing a filtering function or operation (e.g., SQL filtering operation) of its outer sub-query by a qualification or qualify operation (e.g., SQL qualification operation) associated with at least one windowing function of at least one of its inner sub-queries. It will be appreciated that this representation can yield a modified query that would be equivalent (e.g., semantically equivalent) to the original nested query as its execution can produce the same results when optimization rules are applied.