View of Data

A database system is a group of linked files and a collection of software tools that let users access and alter these files. A database system's ability to give users an abstract view of the data is one of its main goals. In other words, the system hides some information about the data's maintenance and storage. The system must retrieve data effectively in order for it to be usable. Designers have chosen intricate data structures to represent data in databases due to the necessity for efficiency. Due to the fact that many database system users lack computer skills, developers often hide complexity from users by layering on abstractions to make system interactions easier for users.

Each of these levels has a perspective that is described by a schema. A schema is a description of the records and relationships present in the view. It can be thought of as an outline or plan. Additionally, the schema explains how entities at one level of abstraction can be translated to the following level. The database schema refers to the overall layout of the database. A database schema has various informations included within itself, some of them are:

• Characteristics of data items, such as entities and attributes,
• Relationships and logical structures between various data points.
• Storage representation format.
• Integrity criteria like physical authorization and political back-up.

Since each view is specified by a schema, the database contains a number of schema that are divided into three layers of data abstraction, or views. Physical schema is found at the lowest level, conceptual schema is found in the middle, and a subschema is found at the highest level. One physical schema, one conceptual schema, and various sub-schemas are often supported by database systems.

External or view level:
The External level, also known as the View level, is the highest level of database abstraction and only includes those parts of the database that are relevant to users or application programs. There may be any number of user views for a particular global or conceptual view, some of which may be identical.
An external schema, sometimes known as a subschema, is used to describe each external view. The definition of the logical records and relationships in the external view make up the external schema. The process of getting the items in the external view from the conceptual view is likewise included in the external schema. Entities, characteristics, and relationships are among the objects.

Logic level or conceptual level:
All database entities and their relationships are present at this level of database abstraction. The entire database is represented by a single conceptual view. The conceptual schema establishes this conceptual picture. It describes every record and relationship found in the conceptual view, and thus every record and relationship found in the database. Each database has a single conceptual schema. The process of obtaining the items in the conceptual view from the objects in the internal view is also included in this schema.
At this level, the description of the data is in a format separate from its actual physical form. Additionally, it has elements that outline the tests to maintain the consistency and integrity of the data.

Physical or Internal Level:
This view, which is closest to the actual physical storage technique being used, is found at the lowest level of abstraction. It outlines the data structures and access techniques the database will employ, as well as how the data will be stored. The internal schema, which includes the specification of the stored record, the way the data fields are represented, and the access aids utilized, expresses the internal view.
To see the different levels of a database's structure, use the three level schema architecture. The three schemas are only descriptions of the data; only the physical level is where the data actually exists. Only its own external schema is referred to by each user group. A request specified on an external schema must be converted by the database management system (DBMS) into a request against the conceptual schema, which must then be converted into a request on the internal schema for database processing. It is known as mapping to transform requests and results between levels.
In the three-schema architecture, mappings connect the schemas at the three different levels. Each level's mappings handle the association of data objects, which may have totally different syntaxes. Between the conceptual and external schemas, there is a logical mapping, and between the internal and conceptual schemas, there is a physical mapping.