The codebase for Microsoft SQL Server originated in Sybase SQL Server, and was Microsoft's entry to the enterprise-level database market, competing against Oracle, IBM, and Sybase. The first version was SQL Server for OS/2 (about 1989) which was essentially the same as Sybase SQL Server 4.0 on Unix, VMS, etc.
About the time Windows NT was coming out, Sybase and Microsoft parted ways and pursued their own design and marketing schemes. Later, Sybase changed the name of its product to Adaptive Server Enterprise to avoid confusion with Microsoft SQL Server. Until 1994 Microsoft's SQL Server carried three Sybase copyright notices as an indication of its origin.
A stripped down version of Microsoft Server known as MSDE (Microsoft SQL Server Desktop Engine) is also available for use in products such as Visual Fox Pro, MS Access, MS Web Matrix, and other products. MSDE has a limit of 2 GB, 8 connections, and comes with no tools to administor it.
MS SQL Server uses a variety of SQL called T-SQL, or Transact-SQL, a superset of SQL-92 (The ISO standard for SQL, certified in 1992). T-SQL mainly adds additional syntax for use in stored procedures, and affects the syntax of transactions support. (Note that SQL standards require atomic, consistent, isolated, durable transactions.) MS SQL Server and Sybase/ASE both communicate over networks using an application-level protocol called Tabular Data Stream (TDS). The TDS protocol has also been implemented by the FreeTDS project ([1]) in order to allow more kinds of client applications to communicate with MS SQL Server and Sybase databases.
A computer worm named the SQL slammer worm which exploits a security vulnerability in this system was discovered in January 2003, and caused a large Internet slowdown on January 24.Possibly this worm caused the largest malfunction in Internet functionality since the infamous Morris worm November 2, 1988.
In September 2001, SQL Server had ~14% of the commercial database market, according to Gartner.
Unlike Oracle, Microsoft's SQL Server does not perform row level locking properly. This is why when you hear about "hundreds of users" on a server, you learn on investigating that it is an application that manages the connections in a way that no two users are running queries at the same time. Otherwise, the system locks up solid, sometimes for 30 minutes or more. Microsoft sales people lied about this lack of record level locking (right in front of me to an audience). They just figured if they told people they had record level locking, people would believe them. Lots of people do now. But it just does not exist, and a quick experiment on a server can easily prove it.