Database can he viewed at three levels. The levels form three-level architecture and are depicted by three models. The models refer to the permanent structure of the database, not to the data that is stored at any given moment. A database model is a collection of tools. It may include a type of diagram and specialized vocabulary for describing the structure of the database.
The purpose of the three-level architecture is to separate the way the database is physically represented from the way the user thinks about it.
External Level:
The external level consists of different external views of the database. Each external view represents the view of a particular user about the system. Different users interact with the system in different ways. Each user is interested in only some of the attributes and relationships. A database stores many attributes and entities but different users may not be aware of all entities and attributes.
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), while another 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 database but created when needed.
Logical or Conceptual Level:
The logical or conceptual level describes the data stored in the database and the relationships among the data. This is the middle-level view in the three-level architecture. This level contains the logical structure of the entire database as seen by the DBA. It is complete view of the data requirements of the origination that is independent of any storage configuration. The theoretical level represents:
• All attributes, entities, and their association
• Constraint on the data
• Semantic information regarding the record data
• Protection and integrity information
The conceptual leve1 supports each external view, it means that any data required by different users must he deliverable from the conceptual level However, this level must not contain any storage-dependent details.
Internal or Physical Level:
This level describes the physical representation of the database on the computer. It describes how the data is stored in database.
The internal level covers the physical implementation of the database to achieve optimal run-time performance and storage utilization. It covers the data structures and file organization used to store data on storage devices.
It works with the operating system access methods like file management techniques for storing and retrieving data records, to place the data on the storage devices, build the indexes, retrieve the data, and so on.
Three Level Architecture
The internal level is concerned with such things as:
• Storage space allocation for data and indexes
• Record description for storage (with stored size for data items)
• Record placement
• Data compression and data encryption techniques
comment closed