最開始的文件系統，只有一個文件夾，所有文件都扔進去，這個時候會有一個限制：文件不能有重復的命名，但后來文件系統發明了“文件夾嵌套”，只要不在同一個目錄下，“文件名”或者“文件夾名字”是可以使用同樣的名字的
數據庫都會做這樣的划分，根據最正宗的SQL標准，是這樣划分的:Cluster > Catalog > Schema > Table
但並不是所有的“數據庫供應商”都按照這個標准，比如MySQL的show databases命令，卻發現infromation_schema都出來了，因為對於MySQL而言，“database 數據庫” 和 “schema 模式” 是同一件事，所以MySQL可以擁有很多個數據庫，但是對於Oracle，卻只有一個orcl數據庫。具體區分如下表
In MySQL:
- server instance == not identified with catalog, just a set of databases
- database == schema == catalog == a namespace within the server.
- user == named account, who is can connect to server and use (but can not own - no concept of ownership) objects in one or more databases
- to identify any object you need (database name + object name)
In Oracle:
- server instance == database == catalog == all data managed by same execution engine
- schema == namespace within database, identical to user account
- user == schema owner == named account, identical to schema, who can connect to database, who owns the schema and use objects possibly in other schemas
- to identify any object you need (schema name + object name)
詳細閱讀：
1.What's the difference between a catalog and a schema in a relational database?
2.Relationship between catalog, schema, user, and database instance