Yeah, this can get tricky… I’d normally try an open join but…varies depending on what you’ve done already, where you need to join, want to show in your BAQ.
Off the top of my head I’d think you’d use a subquery but again… will depends on what your BAQ structure looks like so far.
If you search the site you might find a few examples… this one is for the Job tables but basically the same for Part tables except you won’t have to worry about the Assembly.