使用MySQL(和PHP)搜索可用性吗？

我有两个MySQL(MyIsAm)表,分别代表出租单位和预订：

> LettingUnits(ID,名称等)
> LettingUnitBookings(ID,F_LU_ID,开始,结束)

其中F_LU_ID是该单元的外键.

搜索特定时段内可用单位的最佳方法是什么？搜索通过开始,结束和持续时间.

>开始=最早开始预定
>结束=预订的最后结束
>持续时间=预订持续时间

我想知道是否有可能在MySQL中执行此操作,但是如果不能,那么最好的方法是在PHP中执行此操作.

例

在回答下面的答案时,我觉得一个例子将有助于解释问题.

一个LettingUnit：

>(123,“ Foo Cottage”)

一些LettingUnitBookings：

>(400,123,01/01/09,05/01/09)-5天预订
>(401,10/01/09,20/01/09)-10天预订
>(402,25/01/09,30/01/09)-5天预订

如果我们搜索：

>开始= 01/01/09
>结束= 01/02/2009
>持续时间= 5(天)

然后,我们要显示该单元.因为在搜索范围内可以预订5天.

如果持续时间为10,则该单元将不会显示,因为在搜索范围内没有连续10天的未预订天数.

SELECT t.*,DATEDIFF(t.LatestAvailable,t.EarliestAvailable) AS LengthAvailable
FROM 
   (SELECT u.*,COALESCE(b1.End,@StartOfWindow) AS EarliestAvailable,COALESCE(b2.Start,@EndOfWindow) AS LatestAvailable
    FROM LettingUnits u
    LEFT OUTER JOIN LettingUnitBookings b1
      ON (u.ID = b1.F_LU_ID AND b1.End BETWEEN @StartOfWindow AND @EndOfWindow)
    LEFT OUTER JOIN LettingUnitBookings b2
      ON (u.ID = b2.F_LU_ID AND b2.Start BETWEEN @StartOfWindow AND @EndOfWindow
          AND b2.Start >= b1.End) -- edit: new term
    ) AS t
LEFT OUTER JOIN LettingUnitBookings x
  ON (t.ID = x.F_LU_ID AND x.Start < t.LatestAvailable AND x.End > t.EarliestAvailable)
WHERE x.ID IS NULL AND DATEDIFF(t.LatestAvailable,t.EarliestAvailable) >= @WindowSize;

输出为：

+-----+-------------+-------------------+-----------------+-----------------+
| ID  | Name        | EarliestAvailable | LatestAvailable | LengthAvailable |
+-----+-------------+-------------------+-----------------+-----------------+
| 123 | Foo Cottage | 2009-01-05        | 2009-01-10      |               5 |
| 123 | Foo Cottage | 2009-01-20        | 2009-01-25      |               5 |
| 456 | Bar Cottage | 2009-01-20        | 2009-01-31      |              11 |
+-----+-------------+-------------------+-----------------+-----------------+

用EXPLAIN对此进行分析表明,它很好地利用了索引：

+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra                   |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL    | NULL    | NULL  |    9 | Using where             |
|  1 | PRIMARY     | x          | ref    | F_LU_ID       | F_LU_ID | 8       | t.ID  |    2 | Using where; Not exists |
|  2 | DERIVED     | u          | system | NULL          | NULL    | NULL    | NULL  |    1 |                         |
|  2 | DERIVED     | b1         | ref    | F_LU_ID       | F_LU_ID | 8       | const |    0 |                         |
|  2 | DERIVED     | b2         | ref    | F_LU_ID       | F_LU_ID | 8       | const |    0 |                         |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+

与@martin clayton的given解决方案的EXPLAIN报告进行比较：

+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
| id | select_type  | table               | type   | possible_keys | key     | key_len | ref  | rows | Extra                           |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
|  1 | PRIMARY      | lu                  | system | PRIMARY,ID    | NULL    | NULL    | NULL |    1 |                                 |
|  1 | PRIMARY      | <derived2>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using where                     |
|  2 | DERIVED      | <derived3>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using temporary; Using filesort |
|  2 | DERIVED      | <derived5>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using where; Using join buffer  |
|  5 | DERIVED      | LettingUnitBookings | ALL    | NULL          | NULL    | NULL    | NULL |    3 |                                 |
|  6 | UNION        | LettingUnitBookings | index  | NULL          | F_LU_ID | 8       | NULL |    3 | Using index                     |
| NULL | UNION RESULT | <union5,6>          | ALL    | NULL          | NULL    | NULL    | NULL | NULL |                                 |
|  3 | DERIVED      | LettingUnitBookings | ALL    | NULL          | NULL    | NULL    | NULL |    3 |                                 |
|  4 | UNION        | LettingUnitBookings | index  | NULL          | F_LU_ID | 8       | NULL |    3 | Using index                     |
| NULL | UNION RESULT | <union3,4>          | ALL    | NULL          | NULL    | NULL    | NULL | NULL |                                 |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+

通常,您要避免强制使用文件排序或使用临时的优化计划,因为这些计划会降低性能.使用GROUP BY的查询几乎可以肯定会导致这种优化,至少在MySQL中如此.

（编辑：李大同）

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