- PRQL is focused on analytical queries and it does not support DML (yet at least, see https://github.com/PRQL/prql/issues/1092). Thus there is no UPDATE or "upsert"

- What is merge? How is it different from join? How is it different from union?

- SQL does define `information_schema`, which can be used to list tables and columns. We had ideas about introspection and listing tables, but this is not on the roadmap yet. A probable outcome here will be improvements to tooling so you will be able to find tables via autocomplete (LSP).

- There are no plans for EXPLAIN