Euan Garden has an excellent article on his blog about the history of SQL Server
I got all the history I know mostly from the book Inside SQL Server 2000 and from the SQL Down Under podcast with Kalen Delany
This is how it started:
In 1987 Microsoft and Sybase started a partnership to build/sell a Database Management System, based on the then (yet to be released) Sybase DataServer product. Sybase would have the rights to the product on the Unix/Mini Platform and Microsoft would have the rights on the fledgling OS2, and any other OS developed by Microsoft.
So go ahead and read the rest here, you will not regret it