Return every compound registered under exactly three CAS numbers, with its name and how many it has, ordered by name. SQL counts rows in cas_numbers and filters the count with HAVING; Mango reads the length of the cas array with $size. Thirty-eight compounds answer this. Write it in whichever language you like, then write it in the other.
A intermediate exercise, answered in SQL or as a Mango query. 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.