AWK in shell script - How to compare and merge TWO files based on a shared key (2 common fields)?

Question

Could some one please help to compare two files, i have used the below command but i couldnt succeed on it,

awk -F, 'NR == FNR {a[$1,$2]; next} (($1,$2) in a )' temp1.dat temp2.dat

Here is my need, need to compare first two fields in the below two dat files and merge the result as expected in file3(first field, second field, 3 field of temp1.dat, 3 field of temp2.dat)

File1:temp1.dat

A, AB,100
B,BB,200
C,CC,300

File2:temp2.dat

A,AB,10
C,CC,30
D,DF, 4

File3 :output

A, AB,100,10
C,CC,300,30

Answer 1

awk -F, 'BEGIN{OFS=","}FNR==NR{a[$1$2]=$3;next}($1$2 in a && $3=$3","a[$1$2])' file2 file1

tested below:

> cat file1
A,AB,100
B,BB,200
C,CC,300
> cat file2
A,AB,10
C,CC,30
D,DF,4
> awk -F, 'BEGIN{OFS=","}FNR==NR{a[$1$2]=$3;next}($1$2 in a && $3=$3","a[$1$2])' file2 file1
A,AB,100,10
C,CC,300,30
> 

• FNR==NR{a[$1$2]=$3;next} is applied for the first file file2
• it says untill FNR==NR execute teh block of code.
• FNR=line number of the current file
• NR= line number of the total lines of two files.
• so after the above statement an associative array with index as $1$2 and value as $3.
• Now ($1$2 in a && $3=$3","a[$1$2]) this executes for FNR!=NR.where in it checks for index $1$2 exists as an index in the array and then teh second condition is changing the 3rd field of file1 to $3=$3","a[$1$2]so now $0 contains the common($1$2) lines which changed 3rd field in them.

similar logic has to be written for four files also.

Answer 2

Try:

awk -F, '{i=$1 SUBSEP $2} NR==FNR{A[i]=$3; next} i in A{print $0,A[i]}' file2 file1