At Microsoft Connect(); 2016, it was announced that MS SQL Server was available for Linux (Ubuntu and RedHat) and MacOS. This was a real surprise and I’m not sure if its free or not. It was an easy installation on Ubuntu 16 running on HyperV on a Windows 2012 server.
The installation procedure for MSSQL and SQL Server Tools are at:
After you install the software, a brief tutorial on using it from the command line is here, but I prefer using SQL Server Management Studio (see below).