Wikipedia
FactorySQL
FactorySQL is an OPC based Middleware product by Inductive Automation that bridges the gap between industrial PLCs and SQL Databases. FactorySQL is a drag and drop .NET application that runs as a Windows service. FactorySQL version 1.0 was released in 2003 and deprecated at version 4 in 2010 with the release of Ignition Inductive Automation offers the OPC-UA module for free. As of January 2013, FactorySQL is still being sold and supported.