Enterprise MySQL (the version you pay Oracle to license) has a PAM module that allows for LDAP authentication: https://dev.mysql.com/doc/refman/5.5/en/pam-pluggable-authentication.html
MariaDB (a binary compatible version of MySQL developed by Monty) has an open source PAM module available for it: http://kb.askmonty.org/en/pam-authentication-plugin/
I do not have hands on experience with either - I present them only as features I have heard of but not tested or used myself.