There are no absolute rights and wrongs I suppose, but I'd have to say no. I haven't encountered any situation where I would expect a PM to answer a technical question for developers on how to join data. More likely the PM's job would be to suggest who else the developer ought to approach with that question, e.g. a more senior tech, a data modeler/architect, or SME.
Here are some possible reasons why that sort of question might be coming to you:
- Incomplete analysis or design. The developer has been left to figure it out and doesn't know how to go about analysis and design.
- Inadequate communication with users, stakeholders or SMEs. At the very least the developer should have been told or should have someone to ask what the end result ought to look like. Once the expected results are known then they can try out different solutions.
- The development team isn't sufficiently empowered, communicative or accountable for delivery. Didn't the developer ask their colleagues for advice? If no one on the team knew better than to ask you then that's a cause for concern. The team as a whole ought to own that kind of problem.
It seems likely that the last point is the most important thing you should fix: make sure the development team is sufficiently empowered and self-supporting.