Return one row per supplier with the number of catalogue entries they list, busiest first. A listing is one supplier's entry for one compound, so counting rows in catalog_entries counts listings. Three rows answer this, over 1049 listings in all. The reference supplier is not a shop: it is the record the catalogue itself keeps, and it accounts for 769 of them.
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.