characteristics of the database approach are (1) use of a catalog to store the database description (schema) so as to make it self describing, (2) insulation of programs and data (program-data and program-operation independence), and (3) support of multiple user views.so that we specify an architecture for database systems, called the three-schema architecture.To help achieve and visualize these characteristics.

The aim of the three-schema architecture, is to separate the user applications from the physical database. In this architecture, schemas can be defined at the following three levels:
1. The internal level has an internal schema which describes the physical storage structure of the database. The internal schema uses a physical data
model and describes the complete details of data storage and access paths for data base.

2. The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hides the details of physical storage structures and concentrates on describing entities,data types, relationships, user operations, and constraints. Usually, a representational data model is used to describe the conceptual schema when a database system is implemented. This implementation conceptual schema is often based on a conceptual schema design in a high-level data model.
3. The external or view level includes a number of external schemas or userviews. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group. As in the previous level, each external schema is typically implemented using a representational data model, possibly based on an external schema design in a high-level conceptual data model.
The three-schema architecture is a convenient tool with which the user can visualize the schema levels in a database.Most DBMSs do not separate the three levels completely and explicitly, but they support the three-schema architecture to some extent. Some older DBMSs may include physical-level details in the conceptual schema. The three-level ANSI architecture has an important place in database technology development because it clearly separates the users’ external level, the database’s conceptual level, and the internal storage level for designing a database.It is very much applicable in the design of DBMSs, even today. In most DBMSs that user views, external schemas are specified in the same data model that
describes the conceptual-level information (for example, a relational DBMS like
Oracle or SQLServer uses SQL for this).
the three schemas are only descriptions of data; the actual data is storedat the physical level only. In the three-schema architecture, each user group refers to its own external schema. Hence, the DBMS must transform a request specified on an external schema into a request against the conceptual schema, and then into a request on the internal schema for processing over the stored database. If the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user’s external view. The processes of transforming requests and results between levels are called mappings. These mappings may be time-consuming, so some DBMSs—especially those that are meant to support small databases—do not support external views.