In a database management system (DBMS), a schema is a representation or a specification of the structure, the relationships, and the constraints of the data in a database. A schema is used to define the structure and the organization of the data in a database, and to provide a mechanism for accessing and manipulating the data.

There are usually multiple levels or views of a schema in a DBMS, which represent different perspectives or aspects of the data in a database. These levels or views of a schema are called schema levels, and they are used to provide different levels of abstraction or detail of the data in a database.

Mappings between schema levels are used to relate or connect the different levels or views of a schema in a DBMS, and to provide a mechanism for accessing and manipulating the data across different levels of abstraction or detail. Mappings between schema levels are used to ensure the consistency, the integrity, and the reliability of the data in a database, and to provide a mechanism for integrating and coordinating the data across different levels of the schema.

There are several reasons why we need mappings between schema levels in a DBMS:

• Data integration: Mappings between schema levels are used to integrate or combine the data from different levels or views of a schema in a DBMS, and to provide a consistent and coherent view of the data across different levels of abstraction or detail. Mappings between schema levels are used to ensure that the data from different levels of the schema is integrated and coordinated, and that it is consistent and coherent.
• Data abstraction: Mappings between schema levels are used to abstract or hide the details or the complexity of the data in a database, and to provide a simplified or a higher-level view of the data. Mappings between schema levels are used to provide different levels of abstraction or detail of the data in a database, and to allow the users or the applications to access and manipulate the data at the appropriate level of abstraction or detail.
• Data security: Mappings between schema levels are used to control or restrict the access or the modification of the data in a database, and to provide a mechanism for ensuring the confidentiality, the integrity, and the availability of the data. Mappings between schema levels are used to provide different levels of access or privilege to the data in a database, and to allow the users or the applications to access and manipulate the data according to their role or their permission.
• Data optimization: Mappings between schema levels are used to optimize or improve the performance or the efficiency of the data in a database, and to provide a mechanism for accessing and manipulating the data in the most effective or the most efficient way. Mappings between schema levels are used to optimize or index the data in a database, and to provide a mechanism for accessing and manipulating the data using the most appropriate or the most efficient techniques and technologies.

In summary, mappings between schema levels are used to relate or connect the different levels or views of a schema in a DBMS, and to provide a mechanism for accessing and manipulating the data across different levels of abstraction or detail. Mappings between schema levels are used to ensure the consistency, the integrity, and the reliability of the data in a database, and to provide a mechanism for integrating and coordinating the data across different levels of the schema.