In which os sql server works?

SQL server is a database developed by the Microsoft Corporation. The database is built to exclusively work on Microsoft Operating systems like Windows XP, Windows 2000, Windows Vista etc.

It works on all Windows based platforms, but it does depend on the Edition of SQL Server you are going to implement. For example you can run SQL Server Express on Windows Server, but you cannot run SQL Server Enterprise on Windows XP. It will only run on Windows Server systems.