Uninstalling MSSQL Server instances via Plesk Installer
Plesk Installer allows installing MSSQL Server but not uninstalling.
Instances can be uninstalled programmatically. See for example: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/35af044d-2c98-4617-838f-06a524ea00fb/uninstalling-a-sql-instance-programmatically?forum=sqlsetupandupgrade
Marco Marsala commented
There are bugs too.
Doing these steps:
1) Install MSSQL2014 via Plesk Installer
2) Install MSSQL2012 via Plesk Installer
3) Remove MSSQL2014 following the KB: https://support.plesk.com/hc/en-us/articles/213951225-How-to-uninstall-an-Existing-Instance-of-SQL-Server
Will result in MSSQL2012 instance not being shown anymore under Tools & Settings > Database Servers. Even worse, trying to migrate a MS SQL Server 2012 Database with Plesk Migrator from a target server will result in error message: "The component mssql2012 is not installed on the destination server. mssql2012 will be unavaiable for all restored objects. Please install mssql2012. Note that this may take a few minutes."