I replied to a question in a forum today about intercepting and changing a SQL query, before it reaches SQL Server. I started typing a reply, and realized that... wow... "Does this take me on a trip down memory lane, or what!". So, just for fun, here is what I wrote (slightly modified):
There used to be products on the market, called "Database Gateways", something that looks like one type of DBMS but actually accepts the SQL queries and submits them to some other type of DBMS. Basically an "in between" piece of software. For instance I used to work with such a product, which looked like a SQL Server but submitted the queries to IBM's mainframe DB2.
That market pretty much disappeared around when ODBC was born. Yes, we are talking ancient here, around 1992.
There was a leftover from this time period in SQL Server, called "Open Data Services", which was basically an API allowing you to write something that looks like a SQL Server. There was even a sample app which allowed you to intercept the SQL queries and then re-submit them to a SQL Server.
This API was in turn transformed over time to what became known as "extended stored procedures", which in turn was deprecated with SQL Server 2005 and the SQL CLR support.