Mapping in database

 Topic Outline


Mapping in database
  • From Conceptual to Internal level
  • From External to Internal level


Mapping

Mapping is a process of transforming requests and result from one level to another level. In this process, the data present at one level is related to the data at another level. The mapping defines the three levels. The database management system is responsible for mapping between these three types of schemas in the database. 
There are two types of mapping in the database.

1: From Conceptual to Internal Level

2: From External  to Conceptual Level

1: Conceptual/Internal Mapping

The conceptual/internal mapping lies between the conceptual view and the stored database.  It specifies how conceptual records are stored and retrieved from the storage device. This mapping relates conceptual schema with internal schema. 

If the stored database structure is changed, then conceptual/internal mapping must be changed so the conceptual schema remains consistent. It is the database administrator's responsibility to manage such changes. 

Diagram

Shape 

External/Conceptual and Conceptual/Internal Mapping


2: External/Conceptual Mapping

The external/conceptual mapping role is to define the correspondence between a specific external view and a conceptual view for representing the data. This mapping relates external schema with conceptual schema. The difference between the external level and conceptual level is similar to the difference between conceptual view and stored database. 

For example, 

The fields can have different data types, the fields and record names can change, Many conceptual fields can be combined into a single field. 

Any number of external views can exist at the same time in this mapping. Any number of users can share an external view. 

The below diagram shows the representation of data at different levels of database architecture. The data is stored at the physical level in binary format. This data is separate from the internal view of data. Data is prefixed with Block Header and Record Header.

Block header is used with the group of records.

Record header is used with every record.

Shape 

Representation of data at different levels of database architecture




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