`FULL OUTER JOIN` is the secret to diff'ing table sources. `MERGE` is just a diff operation + insert/update/delete statements to make the target table more like the source one (or even completely like the source one).

`FULL OUTER JOIN` is essential to implementing `MERGE`. Granted, one could implement `MERGE` without implementing `FULL OUTER JOIN` as a public feature, but that seems silly.

Sadly, the SQLite3 dev team specifically says they will not implement `FULL OUTER JOIN`[0].

Implementing `MERGE`-like updates without `FULL OUTER JOIN` is possible (using two `LEFT OUTER JOIN`s), but it's an O(N log N) operation instead of O(N).

The lack of `FULL OUTER JOIN` is a serious flaw in SQLite3. IMO.

  [0] https://www.sqlite.org/omitted.html