Hm, maybe it's because I'm so used to SQL, but I had to lol about the yaml example, I think it's quite nasty to read and I'm glad that SQL looks different.
SELECT: [num, name]
FROM: [customers]
WHERE EXISTS:
SELECT: [name]
FROM: [orders]
WHERE:
AND:
- EQUALS: [customers.num, orders.customer_num]
- LT: [price, 50]
True, it's a little side hustle that's funny but doesn't necessarily make much sense.