Schem定义
A schema is a collection of database objects (used by a user.).
Schema objects are the logical structures that directly refer to the database’s data.
A user is a name defined in the database that can connect to and access objects.
Schemas and users help database administrators manage database security.
schema为数据库对象的集合，类似于包名，如：tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links。通常一个用户有一个缺省的schema，用户名与schema的名称一致。当然一个用户可以管理多个schema。
