Once a database system has been designed, it will be possible to identify each type of data item, data aggregate, record and set by a name or code. It will be possible to state which data item types go together to make data aggregate types and record types. A coded set of tables describing this information and stored in the computer system on direct access devices is called a Schema. It is a description of the data structure which s separate from the data itself. The schema describes the areas, their identifiers and page sizes, and indicates how these are related to records and sets. In other systems, a different set of tables is used for this.
The schema, therefore, is the view of the data, the overall logical data structure which is held by the DBMS. Each time a program requires data, the DBMS will look up in the schema for the details of the structure of the data requested. For example, if the program requires. occurrence of a set, the DBMS will look up in the schema which record types are
required, how to find the relevant records given a certain key by the program, and perhaps also which areas the pages containing the relevant data are stored in.
In a database system, it is not always possible to allow programmers to write the data division of their choice for reasons of security or control. It is more useful to provide the programmer with a standard description of the logical data to be used in a particular application. All references to data within the program will be for this description, which is called a Subschema and is similar to the Schema in structure. The DBMS has the job of matching data requests on a subschema and data requests based on the schema.
Database System Architecture
- The architecture was proposed by ANSI/SPARC study group of
- The knowledge of this architecture is extremely useful in describing general database concept and structure of individual system.
- Major purchase of a database system is to provide the users with an abstract view of data, e., the system hides certain things like how the data is stored and maintained. But on the other part, data must be retrieved efficiently in order to make the system usable.
- Since many database end users are not computer trained, the complexity hidden from them in order to simplify their interaction with the system is through three levels of abstraction.
(1) Physical Level (Internal Level)
- This is the lowest level of data which describes how the data are actually stored and organized on the medium. It describes the complete details of data storage and access paths for the database.
In other words, it has the definition of the records stored and methods of representing the data fields.
(2) Conceptual Level (Global Level)
- It deals with data actually stored in the database. It describes the structure of the whole database for users in terms of relatively small structures.
- There is on conceptual scheme per database, which also contains the method of
driving the objects in the conceptual view from the objects in the internal view.
- It describes all the records and relationship included in the conceptual view.
(3) External Level (View Level)
- It describes only.part of the database of concern to application program.
- It consists of definitions of logical records and the relationships in the external view.