MySQLの予約可能性、重複する日付範囲

1. 希望する期間に発生するすべての予約境界（つまり、開始日と終了日）のリストを作成します。

     SELECT date_start AS boundary
     FROM   bookings
     WHERE  date_start BETWEEN @start AND @end
   UNION
     SELECT date_end
     FROM   bookings
     WHERE  date_end BETWEEN @start AND @end
   

2. それに加えて、希望する期間の直前に発生する境界：

   -- [ from part 1 above ]
   UNION
     SELECT MAX(boundary)
     FROM (
       SELECT MAX(date_start) AS boundary
       FROM   bookings
       WHERE  date_start <= @start
     UNION ALL
       SELECT MAX(date_end)
       FROM   bookings
       WHERE  date_end <= @end
     ) t
   

3. この結果とbookingsを外部結合します テーブル、すべての境界を保持しますが、後の同時人数に寄与する場合にのみ予約を含めます 境界：

   FROM bookings RIGHT JOIN (
     -- [ from part 2 above ]
   ) t ON date_start <= boundary AND boundary < date_end
   

4. 各境界の人数を合計します：

   SELECT IFNULL(SUM(quantity),0) AS simultaneous_people
   -- [ from part 3 above ]
   GROUP BY boundary
   

5. 最大値と最小値を見つける：

   SELECT MIN(simultaneous_people),
          MAX(simultaneous_people)
   FROM (
     -- [ from part 4 above ]
   ) t
   

すべてをまとめる：

SELECT MIN(simultaneous_people),
       MAX(simultaneous_people)
FROM (
  SELECT IFNULL(SUM(quantity),0) AS simultaneous_people
  FROM   bookings RIGHT JOIN (
    SELECT date_start AS boundary
    FROM   bookings
    WHERE  date_start BETWEEN @start AND @end
  UNION
    SELECT date_end
    FROM   bookings
    WHERE  date_end BETWEEN @start AND @end
  UNION
    SELECT MAX(boundary)
    FROM (
      SELECT MAX(date_start) AS boundary
      FROM   bookings
      WHERE  date_start <= @start
    UNION ALL
      SELECT MAX(date_end)
      FROM   bookings
      WHERE  date_end <= @end
    ) t
  ) t ON date_start <= boundary AND boundary < date_end
  GROUP BY boundary
) t

sqlfiddle でご覧ください 。