Why is the database schema important
Database schemas and database instances
Regardless of the database model, it is important to distinguish between the description of the database and the database itself. The description of a database is Database schema or called. The database schema is determined during the database design process and changes very rarely afterwards.
The actual data in a database change frequently over time. The database status at a specific point in time, given by the currently existing content and relationships and their attributes, is Database instance called.
The following illustration shows that the database schema can be viewed as a template or blueprint for one or more database instances.
When designing a database, a distinction is made between two levels of abstraction and their corresponding data schemes (conceptual data scheme and logical data scheme, see definitions below).
- Conceptual data schema:
- A conceptual data schema is a system-independent data description, i.e. it is independent of the database and computer systems used. (ZEHNDER 1998)
- Logical data scheme:
- A logical data schema ("logical schema") describes the data in the data description language (DDL = Data Definition Language) of a specific database management system. (ZEHNDER 1998)
The conceptual data scheme is based exclusively on the database application and thus on the real world, but not on the data technology infrastructure (DBMS and computer systems) that may be used for this. and relations are tools for creating a conceptual schema.
When designing a database, the logical schema is derived from the conceptual data schema (see Unit Relational Database Design). At the end of this derivation there is the logical data scheme for a special application and a special DBMS. A DB development system then converts the logical scheme directly into instructions for the DBMS.
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