先决条件 – 属性类型
1.存储属性：
存储属性是物理存储在数据库中的属性。
假设有一个名为 student 的表。有student_id、name、roll_no、course_Id等属性。我们无法使用其他属性推导出这些属性的值。因此，这些属性称为存储属性。
2.派生属性：
派生属性是其值从其他属性计算出来的属性。在学生表中，如果我们有一个名为 date_of_birth 和年龄的属性。我们可以借助 date_of_birth 属性推导出年龄值。
存储属性和派生属性的区别：
| S.NO. | Stored Attribute | Derived Attribute |
|---|---|---|
| 1. | For a stored attribute it is mandatory to be stored in the database. | For a derived attribute it is not mandatory to be stored in the database. |
| 2. | It is not possible to determine the value of a stored attribute. | It is possible to determine the value of a derived attribute with the help of another attribute. |
| 3. | It is an attribute that cannot be derived from any other attribute. | It is an attribute that can be derived from any other attribute. |
| 4. | Stored attribute will be fixed. | Derived attribute will be variable in nature. |
| 5. | Stored attribute is represented by an oval. | Derived attribute are represented by a dotted oval. |
| 6. | Stored attribute saves the CPU’s processing cycle. | Derived attribute if not stored in Relational table, then save storage space. |
| 7. | Stored attribute save data access time. | Derived attribute if it is not stored in the Relational table, then the data access time is longer. |
| 8. | Stored attribute requires constant maintenance to ensure that the derived value is current. | Derived attribute if it is not stored in the Relational table, does not require maintenance, since it is derived from the stored attribute. |
| 9. | Stored attribute does not add coding complexity to queries. | Derived attribute if not stored in the Relational table, adds coding complexity to queries. |
| 10. | Stored attribute data value is readily available. | Derived attribute if not stored in the Relational table, data value is not readily available. |