How can I merge two files?

Question

I have two files

File 1:

7118
7457
7591
7539
8001

File 2:

5003
5008
5011
5026
5028
5029
5031

Output t

Answer 1

awk 'FNR==NR{a[$0];next}{ for(i in a) print i,$0 }' OFS="," file file1