Database Reference
In-Depth Information
In this model, data remains consistent and accurate due to the various levels of integrity
we can impose within the database.
Data can be retrieved either from a particular table or from any number of related tables
within the database. User can view information in unlimited number of ways.
Disadvantage
The relational data model has some disadvantages:
Disadvantage of the relational database was that software programs based on it ran very
slowly. In this model, a relational database system depends on machine performance.
It is not suited to all organizations.
In this model relationship among tables are inherently defined. One has to know the inner
data structure, to familiar with the physical details of relations. In above example, the user
may not be familiar with the inter relationship between the two tables without viewing their
data definition.
2.2 Schemas
A schema is the
physical layout
of tables in a database. In other words, a schema is a spe-
cification of the physical database's information content and logical structure. The structure
could be the overall database or an object inside the database. A table's structure would be
considered a schema because it defines the structure in terms of its columns or attributes.
Think of the logical database design as the architectural blueprints and the physical data-
base implementation as the completed home.
The logical database design describes the size, shape, and necessary systems for a database;
it addresses the informational and operational needs of your business. We then build the
physical implementation of the logical database design, using DBMS software program.
Once tables have been created, set up table relationships, and established the appropriate
levels of data integrity. Now database is complete and ready to create applications that al-
low user to interact easily with the data stored in the database. After this, these applications
will provide timely and accurate information to the user.
Users have different requirements and therefore each different kind of user must be
provided with a schema that is specific to his or her requirements.
A database has three schema levels:
1) a system level,
2) a logical level, and