2.1.1 Data concepts – conceptual, logical, physical data models

1.1.1.      Data concepts

1. Conceptual models

• Conceptual models Provide flexible data-structuring capabilities, Present a “community view”: the logical structure of the entire database.
• The models contain data stored in the database and show relationships among data including: constraints, Semantic information (e.g., business rules), Security and integrity information.
• Conceptual models consider a database as a collection of entities (objects) of various kinds and are the basis for identification and high-level description of main data objects; they avoid details.
• They are database independent regardless of the database you will be using.

2. Physical models

• Physical models are the physical representation of the database, have the lowest level of abstractions, and are how the data is stored.
• They deal with: Run-time performance, storage utilization and compression, file organization and access methods, data encryption.
• Are the physical level – managed by the operating system (OS) Provide concepts that describe the details of how data are stored in the computer’s memory

3. Logical and Physical Data Independence

Data independence refers to the immunity of user applications to changes made in the definition and organization of data. Data abstractions expose only those items that are important or pertinent to the user. Complexity is hidden from the database user.

Data independence and operation independence together form the feature of data abstraction. There are two types of data independence: logical and physical.

4. Logical data independence

A logical schema is a conceptual design of the database done on paper or a whiteboard, much like architectural drawings for a house. The ability to change the logical schema, without changing the external schema or user view, is called logical data independence. For example, the addition or removal of new entities, attributes or relationships to this conceptual schema should be possible without having to change existing external schemas or rewrite existing application programs.

In other words, changes to the logical schema (e.g., alterations to the structure of the database like adding a column or other tables) should not affect the function of the application (external views).

5. Physical data independence

Physical data independence refers to the immunity of the internal model to changes in the physical model. The logical schema stays unchanged even though changes are made to file organization or storage structures, storage devices or indexing strategy.

Physical data independence deals with hiding the details of the storage structure from user applications. The applications should not be involved with these issues, since there is no difference in the operation carried out against the data.

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