Combining two similar queries into one (PostgreSQL)
Use the CASE function:

SELECT c.id,
       CASE WHEN COALESCE(bv.affidavit, '') = '' THEN False
            ELSE True
       END
FROM   component c, base_vee bv
WHERE  c.id IN (SELECT component_id FROM registry_row WHERE registry_id = 199) 
AND    c.id = bv.component_id;