SPSS table merge with extra variable present in both datasets: which value is kept? - merge

Question

I have what I hope is a simple SPSS question. If I conduct a table merge using the syntax below, but both "bigfile" and "smallfile" have values for some variable [say, ChildID], will 'mergefile' have the values for ChildID from smallfile or from bigfile?
Match files files=bigfile
/table=smallfile
/by JoinID.
dataset name mergefile.
execute.
Thank you very much.
-Dan

Answer 1

From the fine manual:
The order in which files are specified determines the order of
variables in the new active dataset. In addition, if the same variable
name occurs in more than one input file, the variable is taken from
the file specified first.
So this should indicate that ChildID should be the values that were in bigfile, for your particular example. Lets demonstrate this to make sure.
data list free /JoinID ChildID X.
begin data
1 1 4
1 1 5
1 1 6
1 1 7
2 2 8
2 2 9
3 3 2
3 3 1
end data.
dataset name bigfile.
data list free /JoinID ChildID Y.
begin data
1 5 4
2 5 8
3 5 2
end data.
dataset name smallfile.
match files file = 'bigfile'
/table = 'smallfile'
/by JoinID.
dataset name mergefile.
list ALL.
Which produces the output.
JoinID ChildID X Y
1.00 1.00 4.00 4.00
1.00 1.00 5.00 4.00
1.00 1.00 6.00 4.00
1.00 1.00 7.00 4.00
2.00 2.00 8.00 8.00
2.00 2.00 9.00 8.00
3.00 3.00 2.00 2.00
3.00 3.00 1.00 2.00
You may also be interested in the rename sub-command (as well as drop and keep) for match files (to prevent over-writing or to specify which file you want the final variables to come from). My workflow typically drops the cases from one of the files, as files won't merge if they are strings of different length.
An example of using the rename and drop sub-commands is below (using the same example data from above). This will allow you to keep the values from the subsequent files if that is what you prefer.
match files file = 'bigfile'
/rename = (ChildId = Old)
/table = 'smallfile'
/by JoinID
/drop Old.
dataset name mergefile2.
list ALL.