alter system and alter database

ALTER SYSTEM

Purpose

Use the ALTER SYSTEM statement to dynamically alter your Oracle database instance. The settings stay in effect as long as the database is mounted.

--一個是對instance實例進行修改

ALTER DATABASE

Purpose

Use the ALTER DATABASE statement to modify, maintain, or recover an existing database.
--一個是對database庫相關進行修改

留個記號，Oracle隨機文檔
Oracle® Database SQL Language Reference 11g Release 1 (11.1) 。