Step 10 of 18 of the SQL tutorial. WHERE runs before the rows are folded and HAVING runs after, which is why a condition on COUNT can only go in HAVING. Here WHERE drops the five other hazard systems, GROUP BY folds the GHS codes, and HAVING keeps the five that at least twenty listings carry. H319 leads at 116 listings, then H315 at 115 and H335 at 104 — eye, skin, airway, the three most cited irritations in this catalogue. Move the >= 20 into WHERE and the engine refuses: misuse of aggregate function COUNT().
It opens on the query SELECT code, description, COUNT(*) AS nb_listings FROM hazard_statements WHERE system = 'ghs-hazard' GROUP BY code HAVING nb_listings >= 20 ORDER BY nb_listings DESC, 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.