Welcome to the community @ZXS! Great to hear that you enjoy n8n!
About your question. Yes you can one workflow from another with the “Execute Workflow” node:
Guess what you describe now as Workflow B is this: Workflow A -> (Workflow U) -> MySQL
How should I setup the Workflow B? It says I can’t activate unless I have a trigger.
Yes, that can not be activated as the workflow itself does not have a trigger. You would have to “move” the trigger (in thise case the Webhook-Node) from Workflow A to workflow B.
Can I send data to Workflow U and get it back?
Yes, that is how the “Execute Workflow” node behaves by default.
I hope that helps!