ARTICLE: DBMS: Data Schemas and Data Independence

The Skelton structure which is used for the representation of the logical view of the database is called the database schema. It has several functions to perform. Defining the organization of the data and the associated relationships are the two most prominent of all of them. In addition to this, the formulation of the constraints which are to be applied in the data is to done by the database schema.

Along with these functions, the definition of the entities and the associated relationships is also one of the purposes of the database schema. The diagrams which are used for the description of the details of the database are called the schema diagrams. The database designers are the ones who take the responsibility of the design of the data schema. In this way, the designers assist the programmers in understanding the database and ultimately make it expedient for all.


  • Physical Database Schema
  • Logical Database Schema

Physical Database Schema:

It is the schema which belongs to the forms of the actual storage of the data such as the indices and the files etc. The physical database schema also defines the way in which the data can be stored on the secondary device.

Logical Database Schema:

All the logical constraints which are needed for the application of the stored data are included in the logical database schema. Under this, the views, tables and the integrated constraints are involved.

Database Instance

Most of the students get confused in the database schema and the database instance. Therefore, it is essential that they get the separate definitions for the two terms. The skeleton form of the database is the database schema. When the system does not have the database, then the database schema is designed. The changes can be introduced in the system unless the database of the system is not operational. But, there is no composition of the data or information in the database schema.

Whereas, the state of the operational database of the system in which the data can be accessed at any time got the term, the database instance. The snapshot of the database is contained in the database schema. The instances of the database get changed with time. Each state in the database instance gets into the valid state, and then they are followed by the constraints, validations, conditions that the database designers impose on the system.

Data Independence

If you need to make any changes in the database system, then it has to be multi-layered. Otherwise, it becomes difficult to do so. This is why the system of the database are kept multi-layered.

There is a lot of data in the database system along with the data of the user. For instance, the database system stores metadata, which is the data about the data. It does this for the location of the data as well as its retrieval. Once the data is stored in the system, modifying and updating the data about the data is difficult. As the needs and the requirements of the system are to be fulfilled, then the DBMS needs to be expanded. It would become a highly sophisticated job and tedious when the entire data is dependent.

The metadata follows the layered structure and the format. It is because when the user makes some changes in the data at one layer, then it must not get affected at any other level. The date being independent is mapped to each other.

Logical Data Independence

The logical data independence stores the information in which the information of the manageable data. It is the data about the database. For instance, the relation or the table are stored in the database and the limitations of the table.

This type of logical data independence liberalizes itself from disk which has the stored data. Making any changes in the format of the data does not mean that it affects the stored disk data.

Physical data independence

The data which is stored in the format of bits, the schemas here are logical. The actual data in the physical independence of the data is stored on the disk. In this format, the system holds power to change the data and that too without laying any impact on the schema and the logical data.

For example, the upgradation or the change in the storage system need the replacement of the hard disks with the SSD. But it does not mean that it would emphasize the logical schemas or the data.

The next topic which comes here is the ER model and the basic concepts. Read on and click the link to equate yourself with the database management system topics which are here comprehended in easy to understand language.