Gennemsnitlig tid til at besvare besked

Det er overflødigt at sige, at jeg hader at arbejde med MySQL :

SELECT AVG(delay_answer)
FROM (SELECT MIN(delay_answer) AS delay_answer
      FROM (SELECT M1.ref, client, admin, TIMESTAMPDIFF(SECOND, date_original, date) AS delay_answer
            FROM messages M1
            INNER JOIN (SELECT ref, MIN(date) AS date_original
                        FROM messages
                        GROUP BY ref) M2
            ON M1.ref = M2.ref AND date > date_original
            WHERE admin <> 0 AND client = 0) x
      GROUP BY ref) y;

Dette returnerer den gennemsnitlige tid (i sekunder), som det tog for en administrator at besvare en besked (oprettet af enhver, ikke nødvendigvis en klient, men dette kan nemt ændres).