Step 13 of 18 of the SQL tutorial. Ask which compounds are distilled under vacuum and SQL walks two foreign keys down to boiling_points. Mango walks the same path inside one document with $elemMatch, nested once per level: a listing, whose boiling points, one of which is under 50 mmHg. Both find 106 compounds. The difference is what comes back — SQL returns one row per boiling point, Mango returns the whole compound and leaves you to find the branch that matched.
It opens on the query SELECT c.preferred_name, c.formula, b.low_c, b.pressure_mmhg FROM boiling_points b JOIN catalog_entries e ON e.id = b.catalog_entry_id JOIN compounds c ON c.id = e.compound_id WHERE b.pressure_mmhg < 50 ORDER BY b.pressure_mmhg, c.preferred_name LIMIT 20, 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.