R：如何对数据框中定义的不同时间段内的数据进行平均？

假设我有一个数据框“data”，其中包含测量变量 x 的时间序列数据：

     date            x
2009/10/01 00:00    10
2009/10/01 01:00    11
2009/10/01 02:00    12
2009/10/01 03:00    13
2009/10/01 04:00    14
2009/10/01 05:00    15
2009/10/01 06:00    16
2009/10/01 07:00    17
2009/10/01 08:00    18
2009/10/01 09:00    19
2009/10/01 10:00    20
2009/10/01 11:00    21
2009/10/01 12:00    22
2009/10/01 13:00    23
2009/10/01 14:00    24
2009/10/01 15:00    25
2009/10/01 16:00    26
2009/10/01 17:00    27
2009/10/01 18:00    28
2009/10/01 19:00    29
2009/10/01 20:00    30
2009/10/01 21:00    31
2009/10/01 22:00    32
2009/10/01 23:00    33
2009/10/02 00:00    34
...

另一个数据框“事件”，具有由开始和结束日期定义的不同时间段：

id        start              stop
1   2009/10/01 02:00    2009/10/01 04:00
2   2009/10/01 07:00    2009/10/01 10:00
3   2009/10/01 08:00    2009/10/01 20:00
...

现在我想获得不同事件中 x 平均值的表格，如下所示：

id  mean.x
1   13
2   18.5
3   25.5

在数据库中，我执行一个简单的 SQL 语句，如下所示：

SELECT a.id, avg(b.x) 
FROM events as a, data as b 
WHERE b.date between a.start and a.stop 
GROUP BY a.id 

我想知道如何在 R 中进行这样的平均？如果数据中有一个 id 列指示哪个数据点属于该事件，我可以使用“聚合”，但我找不到创建此列的方法...

任何建议将不胜感激。

cin

Edit:

输出（数据）：

structure(list(date = structure(c(1254348000, 1254351600, 1254355200, 
1254358800, 1254362400, 1254366000, 1254369600, 1254373200, 1254376800, 
1254380400, 1254384000, 1254387600, 1254391200, 1254394800, 1254398400, 
1254402000, 1254405600, 1254409200, 1254412800, 1254416400, 1254420000, 
1254423600, 1254427200, 1254430800, 1254434400), class = c("POSIXct", 
"POSIXt"), tzone = "Europe/Berlin"), x = 10:34), .Names = c("date", 
"x"), row.names = c(NA, -25L), class = "data.frame")

输出（事件）：

structure(list(id = 1:3, start = structure(c(1254355200, 1254373200, 
1254387600), class = c("POSIXct", "POSIXt"), tzone = "Europe/Berlin"), 
    stop = structure(c(1254362400, 1254384000, 1254420000), class = c("POSIXct", 
    "POSIXt"), tzone = "Europe/Berlin")), .Names = c("id", "start", 
"stop"), row.names = c(NA, -3L), class = "data.frame")

Edit2:

dput（事件2）：

structure(list(id = structure(1:3, .Label = c("AGH", "TRG", "ZUH"
), class = "factor"), start = structure(c(1254355200, 1254358800, 
1254358800), class = c("POSIXct", "POSIXt"), tzone = "Europe/Berlin"), 
    stop = structure(c(1254362400, 1254384000, 1254420000), class = c("POSIXct", 
    "POSIXt"), tzone = "Europe/Berlin")), .Names = c("id", "start", 
"stop"), row.names = c(NA, -3L), class = "data.frame")

尝试这个：

library(sqldf)
sqldf("
    SELECT a.id, avg(b.x) 
    FROM events as a, data as b 
    WHERE b.date between a.start and a.stop 
    GROUP BY a.id 
")