Whilst it is common to use the same LDAP or ActiveDirectory sever for both the authentication store and the datastore, you may need access to data that is not stored there. SQL servers can also be used as either the only datastore or an additional datastore.
Any datastore will need to contain the username field specified on the authentication store. All datastores are queried whether or not the user exists in them.
OpenAthens LA has connectors for both MS SQL and MySQL. For configuration details see: