SQL @@SERVICENAME is one of the Configuration Function, used to return the registry key under which SQL Server is currently running. If it is a default instance, then it will write MSSQLSERVER as the service name, and if it is a named instance, then it will return the instance name. The syntax behind this is
SQL @@SERVICENAME Example
The below code snippet will show the SQL Server service name that we are currently running.
SELECT @@SERVICENAME AS 'Service Name'