Microsoft SQL Server 2016 SP1 is available for the Back End servers of Skype for Business Server 2015

Microsoft SQL Server 2016 SP1 is now supported for use as Back End databases that will work together with Skype for Business Server 2015. 

Notes

• We recommend that you apply SQL Server 2016 SP1 if you experience a performance issue after you install SOL Server 2016 for Back End databases of Skype for Business Server 2015.

• If you are using SQL Server 2008 R2 as a Back End database, you may find the SQL Server database can't be upgraded after you install the July 2018 cumulative update 6.0.9319.534 (CU7) for Skype for Business Server 2015. This issue has been fixed in January 2019 cumulative update 6.0.9319.537 (CU8), please see KB4345985 "Failed to setup database rtcxds" error when you upgrade the SQL Server 2008 R2 back-end database in Skype for Business Server 2015.

• If you are using SQL Server 2008 R2 as a Back End database but you are not able to update to January 2019 cumulative update 6.0.9319.537 (CU8) immediately, you should not apply this update. For more information, please see KB4345985 "Failed to setup database rtcxds" error when you upgrade the SQL Server 2008 R2 back-end database in Skype for Business Server 2015.
  
  To obtain a guidance for deploying a particular fix with July 2018 Cumulative Update 6.0.9319.534 (CU7), please submit a request to Microsoft Customer Service and Support through the following Microsoft website:
  
  http://support.microsoft.com/contactus/?ws=support