Find all C.id where none of the rows of B have is_true='Y'?

There are two tables C and B. They're connected by C.id=B.cid. I need to find all the C.id es where: - None of the rows corresponding to cid has is_true='Y' I've tried various queries but can't wrap my head around any of them. This is one of my ideas. select * from C inner join B on C.id=B.cid where -- loop all rows of B using B.cid -- output if the row has is_true!='Y'