ORIGINAL_DB_NAME (Transact-SQL)
Applies to: 
SQL Server
Azure SQL Managed Instance
Returns the database name specified by the user in the database connection string. This database is specified by using the sqlcmd-d option (USE database). It can also be specified with the Open Database Connectivity (ODBC) data source expression (initial catalog =databasename).
This database is different from the default user database.
Transact-SQL syntax conventions
Syntax
ORIGINAL_DB_NAME ()
Remarks
If the initial database isn't specified, the function returns an empty string.