OpenLDAP server has an abstraction layer for its database back end, allowing it to be replaced to handle not only different databses, but synthetic directory operations such as proxying, redirection, and filtering.   There are many available back ends, one of them is back-sql which, as the name implies, will translate LDAP queries into SQL so you can use MySQL or MSSQL to persist the directory.   Depending on the platform, and back end database, you will find how-tos on the web detailing how to build an OpenLDAP server fronting a SQL database.

-- Bob