Return the five lightest compounds, with their name, their formula and their molecular weight. The name and the formula live in compounds; the weight is computed from the structure, so it lives in structures.molecular_weight, one row per compound, pointing back through structures.compound_id. Order the rows so the lightest comes first, and stop at five.
A beginner exercise, answered in SQL. It is marked by the rows your query returns rather than by how you wrote it, with 3 hints if you want them.
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.