A database schema refers to the organization of the data to create a blue print in order to construct a useful database for an organization. It contains a set of formulas representing the integrity constraints and all these formulas are expressible in the same language. The conceptual schema which is created first is transformed in to explicit mapping and database schema and this describes in which way real world entities are modeled in database. In oracle database system the schema has different connotation. Schema is also used to refer graphical depiction of the database structure and these are generally stored in a data dictionary.
There are four levels of database schema and they are
- Conceptual schema
- Logical schema
- Physical schema
- Schema object
There are some requirements for the schema integration and these requirements influence the detailed structure of schema that are produced. These are given below:
Completeness:
Database schema should be provided with all the information available in the source data.
Minimality:
Database schema is not ideal in the case if any elements of database schema are dropped.
Overlap preservation:
Each of the overlapping elements which are specified in the input mapping are also in a database schema relation.
Normalization:
In the database schema, independent entities and relationships in the source data should not be grouped together in the same relation. Source specific schema elements and overlapping schema elements should not be grouped.
Extended overlap preservation:
Source specific elements associated with a source’s overlapping elements are passed through the database schema.
