在data.table FAQ中，nomatch = NA参数被认为类似于外部连接，但是，我还不能让data.table执行 * full * 外部连接--只能执行右外部连接。
例如：

a <- data.table("dog" = c(8:12), "cat" = c(15:19))

   dog cat
1:   8  15
2:   9  16
3:  10  17
4:  11  18
5:  12  19

b <- data.table("dog" = 1:10, "bullfrog" = 11:20)

    dog bullfrog
 1:   1       11
 2:   2       12
 3:   3       13
 4:   4       14
 5:   5       15
 6:   6       16
 7:   7       17
 8:   8       18
 9:   9       19
10:  10       20

setkey(a, dog)
setkey(b, dog)

a[b, nomatch = NA]

    dog cat bullfrog
 1:   1  NA       11
 2:   2  NA       12
 3:   3  NA       13
 4:   4  NA       14
 5:   5  NA       15
 6:   6  NA       16
 7:   7  NA       17
 8:   8  15       18
 9:   9  16       19
10:  10  17       20

因此，nomatch = NA生成一个右外连接（这是默认的）。如果我需要一个完全连接呢？例如：

merge(a, b, by = "dog", all = TRUE) 
# Or with plyr:
join(a, b, by = "dog", type = "full")

    dog cat bullfrog
 1:   1  NA       11
 2:   2  NA       12
 3:   3  NA       13
 4:   4  NA       14
 5:   5  NA       15
 6:   6  NA       16
 7:   7  NA       17
 8:   8  15       18
 9:   9  16       19
10:  10  17       20
11:  11  18       NA
12:  12  19       NA

这在data.table上可能吗？