Compare one field, Remove duplicate if value of another field is greater - linux

Question

Trying to do this at linux command line. Wanting to combine two files, compare values based on ID, but only keeping the ID that has the newer/greater value for Date (edit: equal to or greater than). Because the ID 456604 is in both files, wanting to only keep the one from File 2 with the newer date: "20111015 456604 tgf"
File 1
Date ID Note
20101009 456604 abc
20101009 444444 abc
20101009 555555 abc
20101009 666666 xyz
File 2
Date ID Note
20111015 111111 abc
20111015 222222 abc
20111015 333333 xyz
20111015 456604 tgf
And then the output to have both files combined, but only keeping the second ID value, with the newer date. The order of the rows are in does not matter, just example of the output for concept.
Output
Date ID Note
20101009 444444 abc
20101009 555555 abc
20101009 666666 xyz
20111015 111111 abc
20111015 222222 abc
20111015 333333 xyz
20111015 456604 tgf

Answer 1

$ cat file1.txt file2.txt | sort -ru | awk '!($2 in seen) { print; seen[$2] }'
Date ID Note
20111015 456604 tgf
20111015 333333 xyz
20111015 222222 abc
20111015 111111 abc
20101009 666666 xyz
20101009 555555 abc
20101009 444444 abc
Sort the combined files by descending date and only print a line the first time you see an ID.
EDIT
More compact edition, thanks to Steve:
cat file1.txt file2.txt | sort -ru | awk '!seen[$2]++'

Answer 2

You didn't specify how you'd like to handle the case were the dates are also duplicated, or even if this case could exist. Therefore, I have assumed that by 'greater', you really mean 'greater or equal to' (it also makes handling the header a tiny bit easier). If that's not the case, please edit your question.
awk code:
awk 'FNR==NR {
a[$2]=$1
b[$2]=$0
next
}
a[$2] >= $1 {
print b[$2]
delete b[$2]
next
}
1
END {
for (i in b) {
print b[i]
}
}' file2 file1
Explanation:
Basically, we use an associative array, called a, to store the 'ID' and 'Date' as key and value, respectively. We also store the contents of file2 in memory using another associative array called b. When file1 is read, we test if column two exists in our array, a, and that the key's value is greater or equal to column one. If it is, we print the corresponding line from array b, then delete it from the array, and next onto the next line/record of input. The 1 on it's lonesome will return true, thereby enabling printing where the previous (two) conditions are not met. This has the effect of printing any unmatched records from file1. Finally, we print what's left in array b.
Results:
Date ID Note
20111015 456604 tgf
20101009 444444 abc
20101009 555555 abc
20101009 666666 xyz
20111015 222222 abc
20111015 111111 abc
20111015 333333 xyz

Answer 3

Another awk way
awk 'NR==1;FNR>1{a[$2]=(a[$2]<$1&&b[$2]=$3)?$1:a[$2]}
END{for(i in a)print a[i],i,b[i]}' file file2
Compares value in an array to previously stored value to determine which is higher, also stores the third field if current record is higher.
Then prints out the stored date,key(field 2) and the value stored for field 3.
Or shorter
awk 'NR==1;FNR>1{(a[$2]<$1&&b[$2]=$0)&&a[$2]=$1}END{for(i in b)print b[i]}' file file2