It uses join, and also a bad one

yours is the first one that doesn't use a join. I didn't use a join either

Specifying multiple tables in FROM clause is the same as making a CROSS JOIN.

Of all the solutions about yours, yours is the first one that doesn't use a join. I didn't use a join either as it was not necessary.

n * n / 2 = 0 for 0 so first condition isn't really needed