先决条件:物理和逻辑数据独立
1.物理数据独立性:
物理数据独立性基本上用于将概念级别与内部/物理级别分开。易于实现物理数据独立性。通过这种独立性,用户可以更改对概念架构有影响的物理存储结构或设备。
物理数据独立性下的更改示例:
2.逻辑数据独立性:
逻辑数据独立用于更改概念方案,而无需更改以下内容:
逻辑数据独立下的更改示例:
物理和逻辑数据独立性之间的区别:
Physical Data Independence | Logical Data Independence |
---|---|
It mainly concern about how the data is stored into the system. | It mainly concerned about the structure or the changing data definition. |
It is easy to retrieve. | It is difficult to retrieve because the data is mainly dependent on the logical structure of data. |
As compared to the logical independence it is easy to achieve physical data independence. | As compared to the physical independence it is easy to achieve logical data independence. |
Any change at the physical level, does not require to change at the application level. | The change in the logical level requires a change at the application level. |
The modifications made at the internal level may or may not be needed to improve the performance of the structure. | The modifications made at the logical level is significant whenever the logical structure of the database is to be changed. |
It is concerned with the internal schema. | It is concerned with the conceptual schema. |
Example: Change in compression techniques, Hashing algorithms and storage devices etc. | Example: Add/Modify or Delete a new attribute. |