How to format the N1QL output for a subquery

SQL++
2

Try this. N1QL allows returning multiple values from a subquery and hence, they’re returned as arrays. When you need/expect just one value, simply get the first value (object) of the array and deference it.

SELECT META().id,
       msg_count,
       click_count,
       notify,
       send_DateTime,
       send_to,
       send_from,
       subject,
       campaign,
       (
           SELECT summary
           FROM Contacts USE KEYS c.campaign)[0].subject AS subject
FROM Contacts c
WHERE _type = "track_request"
    AND ANY b IN c.send_to SATISFIES b IN ['customer@aol.com'] END
ORDER BY send_DateTime DESC