Access to Internal Database through fire

Hello:

The situation is this:
A firewall with 3 NICs, one of them for implementing a DMZ zone, where a public WWW server is sitting. The other two connect to the internal LAN and to Internet.
Such a public WWW server needs to access an SQL database located in the internal LAN.

I would like to know what is the most secure way to implement such an access.

I’ll very much appreciate any recommendations.

Thank you very much in advance,
Orlando.