External Level View

External Level consists of different external views of a database. Each external view is the view of a particular user about the system, Different users conceive the system in different ways. Each user is interested in one Part of the system and ignores other Parts. One user may not be aware of the whole system at all.

Different views may have different representations of the same data. For example, one user may think that dates are stored in the form (month/day/ year). Another user may think they are represented as (year/month/day).

Some views might include virtual or calculated data. Virtual data is the data that is not actually stored in the database but created when needed. For example, the product name and its price can be stored in a database. The total bill can be created as when desired.
Similarly, the marks of different subjects can be stored in a database. The overall grade of the students can be calculated when the result card is prepared.

The external level is described in the external schema. It is also called subschemas. External schemas refer to different views of data.

 

The logical or conceptual level/View

This is the middle-level view in three-level architecture. The logical or conceptual level
describes the data stored in the database. It contains the definition of the data to be stored in
the database. It also contains the rules and information about the structure and type of data.
It is the complete description of data stored in a database. That is why it is also known as
the community view of the database.

This level contains the logical structure of the entire database as seen by DBA. It hides the
details of the physical storage structure. The conceptual level represents the following:

1. All entities, their attributes and their relationship
2. The constraint on the data.
3. Semantic information about the data
4. Security and integrity information

The conceptual level supports each external view. It means that any data required by any user must be available from a conceptual level. The conceptual model is comparatively constant. DBA designs a conceptual model to fulfill the present and future requirements of the organization. If there is any change in the external model, the conceptual model should be able to accommodate that change. It is important because any change in the conceptual model
requires a lot of effort. It also affects other views or levels of the database. The conceptual level is described in conceptual schemas. There is only one schema for one database.

Internal or Physical Level

The internal level is responsible to store data on storage media. Il describes the physical representation of a database on a computer. It describes how the data is stored in a database. It covers the data structures and files organization used to store data on storage devices.

Internal and physical levels are normally considered to be the same. But there is a slight difference between them. The data is stored in binary format on disks. The binary storage method is implemented by the operating system. DBMS partially decides the way data is stored on the disk. This decision of DBMS is based on the specifications of DBA. Additionally, DBMS adds information to the data to be stored. For example, it selects a specific file organization for storing data on a disk. It also creates specific indexes to implement that file system. It uses the same index information for retrieving the data from the disk.

DBMS performs storage space utilization to consume minimum space for storing data.
Data compression can be performed for this purpose. It also applies different data encryption
algorithms to implement security on the data.

The records at the internal level are presented according to the format of schema definition
but the data is not in record format at the physical level. It is in character format. The rules
specified by the schema of record are not enforced at the physical level. Data is managed by
the operating system at the physical level.

The internal level is described in internal schemas. They describe the internal mode. They
contain the definitions of stored records, methods of representation, data fields and indexes
etc. There is only one internal schema for one database.

• Database Environment