I am merging two dataframes using a relational key (incident number and
incident year), but not all the records match up.  I want to be able to
review only the records that cannot be merged for each individual dataframe
(essentially trying to select records from one dataframe using a multi-value
relational key from the other dataframe).  The following code shows what I
am trying to do.  The final two lines of code do not work, but if somebody
could figure out a workable solution, that would be great.  Thanks.
--Markus

incidents = data.frame(
        INC_NO = c(1,2,3,4,5,6,7,8,9,10),
        INC_YEAR = c(2006, 2006, 2006, 2007, 2008, 2008, 2008, 2008, 2009,
2010),
        INC_TYPE = c("EMS", "FIRE", "GAS", "MVA", "EMS", "EMS", "EMS",
"FIRE", "EMS", "EMS"))

responses = data.frame(
        INC_NO = c(1,2,2,2,3,4,5,6,7,8,8,8,9,10),
        INC_YEAR = c(2006, 2006, 2006, 2006, 2006, 2007, 2008, 2008, 2008,
2018, 2018, 2018, 2009, 2010),
        UNIT_TYPE = c("E2", "E2", "E5", "T1", "E7", "E6", "E2", "E2", "E1",
"E3", "E7", "T1", "E7", "E5"))

merged_data = merge(incidents, responses, by=c("INC_NO", "INC_YEAR"))

relational_key = c("INC_NO", "INC_YEAR")

## following does not work, but I want DF of incidents that did not merge up
with responses
incidents[incidents[,relational_key] %in% responses[,relational_key],]

## following does not work, but I want DF of responses that did not merge up
with incidents
responses[responses[,relational_key] %in% incidents[,relational_key],]

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