最开始的文件系统，只有一个文件夹，所有文件都扔进去，这个时候会有一个限制：文件不能有重复的命名，但后来文件系统发明了“文件夹嵌套”，只要不在同一个目录下，“文件名”或者“文件夹名字”是可以使用同样的名字的

数据库都会做这样的划分，根据最正宗的SQL标准，是这样划分的:Cluster > Catalog > Schema > Table 

但并不是所有的“数据库供应商”都按照这个标准，比如MySQL的show databases命令，却发现infromation_schema都出来了，因为对于MySQL而言，“database 数据库” 和 “schema 模式” 是同一件事，所以MySQL可以拥有很多个数据库，但是对于Oracle，却只有一个orcl数据库。具体区分如下表

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)