My superb admin showed me two tricks. He invokes perl and passes it the regular expression from the shell. Perl processes standard input and writes to standard output.
The first one collects all user emails and then it gets their unique email domains. The second one collects values of LDAP attribute companyDomains and again gets their unique values.
mysql -h mysql-server -u user -ppassword db-name -e ‘SELECT email FROM users’ | perl -nle ‘/.*@(.*)/;print $1;’ | sort -u | wc
ldapsearch -x -h ldap-server -D “cn=readonly,o=services” -w password -b o= users “(objectClass=organizationalUnit)” companyDomains | grep -P ^companyDomains: | perl -ple ‘s/^companyDomains: //;’ | grep -Pv ‘^\d’ | sort -u