Return one row per compound with the number of distinct pressures its boiling points were measured at, keeping only compounds with two or more, most pressures first. A boiling point belongs to a listing and carries its own pressure_mmhg, which is why one compound holds several. Count the distinct pressures, not the rows — two listings often repeat 760 mmHg. Fifty-three compounds answer this, and four of them were measured at three pressures.
A intermediate 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.