Process of transforming request and results between three level it's called mapping.
There are the two types of mappings:
- Conceptual/Internal Mapping
- External/Conceptual Mapping
1. Conceptual/Internal Mapping:
- The conceptual/internal mapping defines the correspondence between the conceptual view and the store database.
- It specifies how conceptual record and fields are represented at the internal level.
- It relates conceptual schema with internal schema.
- If structure of the store database is changed.
- If changed is made to the storage structure definition-then the conceptual/internal mapping must be changed accordingly, so that the conceptual schema can remain invariant.
- There could be one mapping between conceptual and internal levels.
2. External/Conceptual Mapping:
- The external/conceptual mapping defines the correspondence between a particular external view and conceptual view.
- It relates each external schema with conceptual schema.
- The differences that can exist between these two levels are analogous to those that can exist between the conceptual view and the stored database.
- Example: fields can have different data types; fields and record name can be changed; several conceptual fields can be combined into a single external field.
- Any number of external views can exist at the same time; any number of users can share a given external view: different external views can overlap.
- There could be several mapping between external and conceptual levels.