SELECT
 events.b_g,
 events.vector,
 SUM(tries) AS 'attempts',
 COUNT(DISTINCT events.observer) AS 'reporters'
FROM
 events

LEFT JOIN otf      ON events.b_g = otf.b_g
LEFT JOIN requests ON events.b_g = requests.b_g
WHERE
 requests.b_g IS NULL
AND
      otf.b_g IS NULL

GROUP BY events.b_g, events.vector
ORDER BY events.b_g, events.vector, 'attempts'