[TheDong]

All consensus does in bitcoin is determine whether a transaction is possible or impossible (avoid double spends).

With a SQL database, this is not an issue because SQL transactions can have constraints preventing said double spend.That is to say, the SQL database can take a lock on the row and ensure no other concurrent transaction modifies it in parallel.

Consensus solves a problem that only exists in a decentralized system.

Since SQL databases are centralized, that problem doesn't exist. Even for multi-master / distributed SQL variants, there's typically a way to wait for a write to commit across N nodes before going on, which solves the problem much more easily.

In short, I think your question makes no sense.