How can I create an Oracle SQL Query that will generate a list of all the Schemas present in a Database?

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You can use the following Oracle SQL Query to generate a list of all the Schemas present in a Database:

SELECT username 
FROM dba_users 
WHERE account_status = 'OPEN' 
AND default_tablespace not in ('SYSTEM','SYSAUX');

This query will give you a list of all the active user accounts in the database that are not associated with the 'SYSTEM' and 'SYSAUX' tablespaces. These accounts represent the schemas in the database.