When a single ship or a fleet is stacked with a starbase, do they help defend the starbase? That is when a enemy ship attacks the starbase parsec does it attack the fleet or ship first or does it attack the starbase?

Is it a viable strategy to have fleets defend starbases, especially in the early game when production is critical, or do I have to upgrade the starbases with atk/def modules that will rarely be used.

I have never had a SB attacked, I have always intercepted the fleets first, or gambled on thier safety.