Reports

There is a simpler syntax:
dtgrouped2[dt2, ':='(Amount1 = Amount1,
                     Amount2 = Amount2),
    on = .(Unique = Unique1, MonthNo > StartDate, MonthNo <= EndDate)]

dtgrouped2
    MonthNo Unique Total Amount1 Amount2
      <int> <char> <int>   <int>   <int>
 1:       1    AAA    10       7       0
 2:       1    BBB     0      NA      NA
 3:       2    CCC     3      NA      NA
 4:       2    DDD     0      NA      NA
 5:       3    AAA     0       3       2
 6:       3    BBB    35      NA      NA
 7:       4    CCC    15      NA      NA
 8:       4    AAA     0       3       2
 9:       5    BBB    60      NA      NA
10:       5    CCC     0      NA      NA
11:       6    DDD   100      NA      NA
12:       6    AAA     0      NA      NA
79530687
