A command that finally came in handy and maybe it might help somebody:

 tar -zxvf public-b-data.tgz --wildcards --no-anchored 'data*' '--keep-old-files

It decompresses a tar file to gz only files that contain a certain expression in the name. In the specific case above you are unpacking from the public-b-data.tgz file only the files (or directories) begining with ‘data’  . The -wildcards expression allows asterisks and -keep-old-files holds the oldest files that already exist in the directory.