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MySQL COALESCE alternative for selecting both result sets for an insert, not either / or?
INSERT INTO notification
       (text, type, target_id, sender_id, recipient_id, data, timestamp, is_unread)
SELECT DISTINCT '$text', 'comment', '$id', '$senderId',
       COALESCE(comment.author_id, stream.author_id),
       '$dataArray','$timestamp', '1'
FROM stream
     LEFT JOIN comment ON comment.target_id = stream.id
WHERE (comment.target_id = '$id' OR stream.id = '$id')
  AND comment.author_id != '$senderId'"
  AND stream.author_id != '$senderId'

select concat('a','b','c');

select concat_ws(',','a','b','c');