You certainly could do a SQL implementation. For my purposes, each of the many queries was going to involve something like at least six lines of logic (it was "find the nth value satisfying X, Y, and Z condition"). Debugging and trying make sure the indices worked correctly for each query looked uglier than creating a whole different structure - my structure was native C++ so there wasn't the translation issues of SQL.

I used a B-tree modified by adding a member variable including the total number of children of each parent. Its so simple a modification I don't know why more implementations don't use it but, it seems they don't.