It's not really implemented yet.

You can build nested queries, doing some extra work, but it's not elegant and will not work for any cases (especially with PostgreSQL) : https://gist.github.com/samonzeweb/b1ae3e2d1f3203ee1722ac4bd...

Some work to do to add proper nested queries into godb, but for a so simple example it doesn't seem to be extremely hard.