Conditional join in data.table?

Question

I have the following data.table, dtgrouped2:

     MonthNo Unique Total
 1:       1    AAA    10
 2:       1    BBB     0
 3:       2    CCC     3
 4:       2    DDD     0
 5:       3    AAA     0
 6:       3    BBB    35
 7:       4    CCC    15
 8:       4    AAA     0
 9:       5    BBB    60
10:       5    CCC     0
11:       6    DDD   100
12:       6    AAA     0

And another table, dt2:

     Unique1 StartDate EndDate Amount1 Amount2
1:     AAA        0        1       7       0
3:     AAA        1        2       5       0
2:     AAA        2        4       3       2

I want to insert Amount1 and Amount2 from dt2 to dtgrouped2 based on the following logic on "Unique" evaluated for each row of dtgrouped2:

filter(StartDate< MonthNo & EndDate>=MonthNo)
then MAX(EndDate)
then insert Amount1 as Amount1 and Amount2 as Amount2

So you can see that the result is different depending on the row. This would be the expected output:

Date    MonthNo Unique  Items   Amounts Amount1 Amount2
Jan       1      AAA    x         10        7   0
Jan       1      BBB    y          2        NA  NA
Feb       2      CCC    x          3        NA  NA
Feb       2      DDD    y         15       NA   NA
March     3      AAA    y         20        3   2
March     3      BBB    x         35       NA   NA
April     4      CCC    x         15       NA   NA
April     4      AAA    y         50       3    2
May       5      BBB    x         60      NA    NA
May       5      CCC    y         70      NA    NA
June      6      DDD    x         100     NA    NA
June      6      AAA    y         20       NA   NA

Answer 1

I suggest to use non-equi joins combined with mult = "last" (in order to capture only the most recent EndDate)

dtgrouped2[, c("Amount1", "Amount2") := # Assign the below result to new columns in dtgrouped2
              dt2[dtgrouped2, # join
                  .(Amount1, Amount2), # get the column you need
                  on = .(Unique1 = Unique, # join conditions
                         StartDate < MonthNo, 
                         EndDate >= MonthNo), 
                  mult = "last"]] # get always the latest EndDate
dtgrouped2

#     MonthNo Unique Total Amount1 Amount2
#  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

The reason that you would need to join dt2[dtgrouped] first (and not the other way around) is because you want to join dt2 for each possible value in dtgrouped, hence allow multiple values in dt2 to be joined to dtgrouped