The full name of the database which uniquely identifies it from any other database. The global database name is of the form "database_name .database_domain", for example, sales.us.example.com.
The database name portion, sales, is a simple name you wish to call your database. The database domain portion, us.example.com, specifies the database domain in which the database is located, making the global database name unique. When possible, Oracle recommends that your database domain mirror the network domain.
The global database name is the default service name of database, as specified by the SERVICE_NAMES parameter in the initialization file.
Oracle Net Services Configuration Overview
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