MySQL answer.
Re: count how many nights each room of an hotel had been rent for a complete year?
[sql]
SELECT SUM(ROOM_FULL_NB_CUST), R.ROOM_ID
FROM T_ROOM R
JOIN TJ_ROM_FULL_CUST P
ON R.ROOM_ID = P.ROOM_ID
WHERE PLN_JOUR BETWEEN '2002-01-01' AND '2002-12-31'
GROUP BY R.ROOM_ID[/sql]
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