We handle this in Fireproof with a deterministic default algorithm, in addition to having a hash-based tamperproof ledger of changes. Fireproof is not SQL based, it is more like CouchDB or MongoDB, but with cryptographic integrity. Apache 2.0 https://use-fireproof.com

In practice during CouchDB's heyday, with lots of heavy users, the conflict management API almost never mattered, as most people can make do with deterministic merges.