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

keys.txt contains "key;value" pairs ("C" is in this example not part of data.txt, but the awk script should still work):

A;30
B;20
C;10

The output should be as follows:

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

Hence, each row in data.txt that contains any key from keys.txt should get the corresponding value appended to the row in data.txt.

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...