Several questions about Alloy model

I have several questions about Alloy model

1. Through support doc read, it seems alloy model (adapter type is "sql") is corresponding to one SQLLite database table. But usually we will join some tables to generate result data to render view. Can i still use alloy model implement this purpose? How can i achieve that?

2. Alloy model "Migration" property, when does the "migration.down" function execute? In support doc say "rollback" will trigger the migration.down function, but i wonder when to trigger the rollback for the model (update failure? or execute under some condition?) as doc does not explain more.