What is SQL Server?
Microsoft SQL Server has had a presence in the marketplace for more than two decades and as a result is a well supported database that offers ease of integration with other Microsoft products. SQL Server has native integration with many components in the Microsoft BI stack which means SQL Server can be used to enable your analytics from data loading (ETL) to reporting. SQL Server has seen renewed focus from Microsoft with the release of SQL Server 2016 which includes new capabilities with a focus on an hybrid approach to on-premise/cloud and advanced analytics such as R.
The characterization here is that with open source you have to choose between a rich feature set (Postgres) and robust performance (MySQL). With SQL Server you have advanced functions, integrations with .NET, integration with R, combined with super fast performance. Only thing is it costs a lot.
SQL Server is still being actively developed today, and SQL Server 2016 includes many long awaited advancements to make it a competitive database option. The new features include always encrypted, dynamic data masking, JSON support, PolyBase, Query Store, row level security. In addition new capabilities such as stretch database that allow you to seamlessly access your historical data regardless if it is on-prem or stretched to Azure cloud, and advanced analytics extensions now allow users to execute scripts written in a supported language such as R have been introduced.