Step 1 of 18 of the SQL tutorial. Name the columns you want, then the table they live in. compounds holds one row per substance, keyed by its primary key id. LIMIT stops the engine after ten rows, however many the table holds. Mango asks the same thing with two keys: an empty selector matches every document, and fields is the projection.
It opens on the query SELECT id, preferred_name, formula, nb_suppliers FROM compounds LIMIT 10, which you can run and change in place.
Ask the same question of a chemical dataset in SQL and in Mango and compare the two answers. The database is a SQLite file your browser downloads once and queries locally, so nothing you write ever leaves your machine — which is also why the tool needs JavaScript.