I guess the appeal of Datalog is the recursion that can be expressed in rules:

    has_descendant(?ancestor, ?descendant) :-
        has_child(?ancestor, ?child),
        has_descendant(?child, ?descendant).

Here, the assumption is that you have explicit `has_child` facts (expressing vertices in a graph, essentially), and the above rule gives you paths of arbitrary length.

In SQL, given a table has_child(parent, child), it is not clear to me how you can get all descendants of a given person, or all ancestors.

Other people talk about recursive extensions to SQL, maybe that provides a way.