Sometimes you just want to know whether some records in a secondary table exist or not. You don‘t need their actual content.
This is where Subqueries come in handy:
Are there any rows in the SFLIGHT table without a corresponding entry in table SPFLI?
SELECT carrid connid INTO (xcarrid, xconnid) FROM sflight AS f WHERE NOT EXISTS ( SELECT * FROM spfli WHERE carrid = f~carrid AND connid = f~connid ).