Using awk how do I combine data in two files and substitute values from the second file to the first file?

Question

Any ideas how to the following using awk?

Two input files, data.txt and keys.txt:

data.txt contains some data:

A;1
B;2
A;3

Answer 1

awk to the rescue!

assumes the second file has unique keys unlike first file (if not you need to specify what happens then)

$ awk 'BEGIN   {FS=OFS=";"} 
       NR==FNR {a[$1]=$2; next} 
       $1 in a {print $0,a[$1]}' file2 file1

A;1;30
B;2;20
A;3;30

ps. note the order of files...