This graphic illustrated the problem with DocDB's Advanced search prior to version 8.5.10. As described then (10/23/07):
Problem Description: I did an advanced search on two topics in the NOvA DocDB, where I wanted to see documents associated with both topics. If neither of the topics were "parent" topics I got a result that seemed reasonable. If I changed the one topic to its "parent" topic--and had the box checked to "Include sub-topics", then I got a null result. This seems very counter-intuitive and suggests something is wrong.
The type of joins being considered at the time are shown in this html file.
In the end we settled on making specialized web pages, such as this list of requirements documents for the NOvA CD3b DOE Review.