Database plays an integral role in the commercial domains, which provides users to store, update and retrieve the data. Database management system provides the system, tools and interfaces which helps to design a database. Logical database design is an important part in the database management procedure. Logical database supplies a set of program having hierarchical structured table entities in between the databases and eases the process of retrieval of data.
Function of logical database
In many ways logical database design is important. The major function of designing the logical database is to reduce the data duplication. Also it simplifies the addition, deletion and update processes retrieval of desired data from database.
Entity in a database can be the name of any object, person, name or thing. Entity set is nothing but the collection of similar entities. For example, a set of all employees of an organisation is an employee set. Likewise, set of all items in a store is an item set. Every members of an entity is described by its attributes. In the given diagram, there are various entities like cars, exams, films etc.
An attribute describes the properties of the entity of which they are associated. Attributes also specifies properties of relationships. For example, if Employee is an entity, then its attribute should be employee name, employee code, etc. The domain of an attribute is the collection of all possible values an attribute can have. The domain of Name is a character string.
Identification of Entities and relationships.
There is no certain algorithms to identify the entities and relationships. When a word statement is used to describe an applications nouns are entities and verbs are relationships.For example, suppose the statement is “Teachers teach courses”. In this statement, ‘Teachers’ and ‘Courses’ are nouns and ‘teach’ is a verb. So Teachers and Courses are entities whereas teach is a relationship.
Direction of a Relationship
The direction of a relationship indicates the originating entity of a relationship. The entity from which a relationship originates is the parental entity and the entity where the relationship terminates is the child entity.The type of relation is determined by the direction of line connecting relationship component and the entity. To distinguish different types of relation, we draw either a directed line or an undirected line between the relationship set and the entity set.
Necessity for finding cardinality
There are certain reasons to find the cardinality.
(a) The identifier of the relationship will be composite is the cardinality is N:M
(b) It will be single if cardinality is 1:M
(c) If an entity has attached to it, not all entities in the set may be present in the relationship.
(d) It will be useful in designing database.
Importance of Relation in Logical Database
The relationship is the most important term in logical database. The features of relation is listed below:
(a) Ease of storage of entity set as flat file in a computer storage.
(b) Sound theory of relations allows systematic design of relational database.
(c) Theory of normalizing relations, reduces the duplication of data.
(d) It reduces the duplication of data.
(e) It tries to eliminate errors in adding, deleting, altering items in a database.
(f) It simplifies retrieval of data.
Normalization of relations
Normalization means organisation of tables and relations in the database. It usually involves in dividing the large tables into smaller one to reduce the data redundancy. A collection of relations relevant for an application constitute a relational database. Relations are normalized to ensure that:
(a) Collection of relations do not unnecessarily hold duplicate data.
(b) When a data item is modified, it is modified in all relations where it appears.
(c) When a data is deleted accidentally, required data is not deleted.
(d) It simplifies the retrieval of required data.
Role of ER diagram Database Design
An ER diagram is the actual blueprint of the database. Its composition must reflect an organisation’s operations if the database is to meet that organisation’s data requirements. It forms the basis for a final check on whether the included entities are appropriate and sufficient. It is also used as a final crosscheck against the proposed data dictionary entries. The completed ER diagram also lets the designer communicate more precisely with those who commissioned the database design. The completed ER diagram serves as the implementation guide to those who create the actual database. In short, the ER diagram is as important to the database designer as a blueprint is to the architect and builder.
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