Microsoft SQL Server is Microsoft’s relational database management system. Microsoft’s own history timeline records that on May 3, 1989, “Microsoft and Ashton-Tate announce the shipment of the Microsoft SQL Server 1.0,” describing it as “a powerful, relational database server for PC-based Local Area Networks (LANs)” and noting that “the product is the result of a joint development effort of Ashton-Tate, Microsoft, and Sybase.”
A history written by a member of the SQL Server team and preserved in Microsoft’s documentation archive fills in the lineage. In 1987 Microsoft and Sybase began a partnership to build and sell a database system based on Sybase’s DataServer product, with Sybase holding rights on Unix and minicomputer platforms and Microsoft holding rights on OS/2 and other Microsoft operating systems. The first OS/2 version shipped in 1989 as Ashton-Tate/Microsoft SQL Server, and after the Ashton-Tate partnership ended it became simply Microsoft SQL Server. The same account explains that Microsoft built its Windows NT version on the stable 4.2 code base while Sybase pursued a separate System 10 line, and that in 1994 the partnership was dissolved so each company could take its product in its own direction.
From SQL Server 6.0 and 6.5 onward Microsoft rewrote and replaced large parts of the original code, and with SQL Server 7.0 (released around 1998 to 1999) it rebuilt the database engine, query processor, and storage engine. Today Microsoft’s documentation describes SQL Server in editions such as Enterprise, Standard, and the free Express, aimed at workloads ranging from small single-machine applications to large mission-critical systems.
Note on dates: this entry uses the May 3, 1989 announcement date recorded in Microsoft’s own history timeline for the shipment of SQL Server 1.0.