Something along these lines: ``` select ifnull(t2.snapshot_id, t1.snapshot_id), t1.param_id, ifnull(t2.param_value, t1.param_value) from SnapshotParameters t1 left join SnapshotParameters t2 on ( t1.param_id = t2.param_id and t2.snapshot_id=2) where t1.snapshot_id=1 ```