What are the advantages and disadvantages of Entity Relationship Diagrams
Answer (1 of 1): If you mean an entity-relationship diagram, a major limitation is that they do not actually describe the processes involved but only display the. Entity Relationship Diagram, also known as ERD, ER Diagram or ER model, is a The ER diagram example below shows an entity with some attributes in it. This thesis investigates the area of conceptual data modeling and looks for Comparisons will be made of their advantages and disadvantages re- hancements to the Entity-Relationship Model and a suggestion for an auto.
Provide very simple and easy to understand structure of the information. Phases of database design process Before designing any database application one must know goals of the system, expectations of the end users and uses of database application in detailed manner. Check all data requirements produced by the previous phase and design the characteristics of known database transactions. The choice of DBMS depends upon factors like technical, economical etc.
Simple abstractions of real world data gives the idea of how the system will work.
What is ER-Model?Advantages and Disadvantages of E-R Model.
Select the data structure and access paths of database files to achieve good performance. Database system can be implemented only after logical and physical design is implemented.
Constructing ER diagram is a first step in designing a database. ER model defines the data elements and relationships among them.
ER data model is based on perception of real world data consisting of entities data items and relationships among those entities.
What is Entity Relationship Diagram (ERD)?
Popular high level conceptual model used for conceptual design of database. Diagrammatic notation associated with ER model. Consists of Entity, Attributes and Relationships. It is a thing in real world with its own independent existence. Student, Faculty May be an object with physical or logical existence.
What Are The Limitations Or Disadvantages Of ERD?
Has its own properties that describe the entity known as attributes. Entity Set Collection of all entities of same type Strong Entity — Entity type having own distinct primary key by which we can identify specific entity uniquely. Empno in Emp table, RollNo in Student table. Represented by single rectangle Weak Entity — Entity type which cannot form distinct primary key from their attributes. These type of entities are dependent on strong entity for primary key.
Some weak entities contain virtual primary key called as Discriminator. Represented by double rectangle 5 Attributes Various properties that describes an entity Attribute value that describes each entity becomes a major part of data stored in database as each entity will have some value for each of its attributes.
Employee entity has name, age, phone etc. Simple Attributes Which cannot be divided into sub parts E. Salary of employee Composite Attributes Which can be divided into sub parts E. EmployeeTenure can be calculated from DateOfJoin Null Attribute Can take a null value when entity does not have a value for it or the value is unknown. Commission attribute in Employee table specifies whether the Employee has commission or not Key Attribute Must have a unique value by which any row can be identified.
One department can have only one manager. One department can have many employees. Books in library issued by students 10 Constraints On Relationships Contd Participation Constraints Total Participation Every object in an entity must participate in relationship.
Indicated by dark or double line between entity and relationship. Every department must have manager. Partial Participation More than one object in an entity may participate in a relationship. Indicated by single line between entity and relationship. Employee works for department. Process of defining a set of subclass of entity type is also called as super class of specialization.
Set of subclasses that forms a specialization is defined on the basis of some distinguishing characteristic of entity in super class. The subclass defined n specialization is attached by lines to a circle which is connected to super class. An attribute applied only to entities of particular subclass is called as specific attribute. Guaranteed Access Rule — Each data item must be accessible without ambiguity by providing table name and its primary key of the row also include its column name to be accessed.
Systematic Treatment of Null Values — Null values are not equal to blank space or zero they are unknown unassigned values which should be treated properly. Such an initial model can also be evolved into physical database model that aids the creation of relational database, or aids in the creation of process map and data flow model.
In this section we will go through the ERD symbols in detail. Studentobject e. Invoiceconcept e. Profile or event e. In ERD, the term "entity" is often used instead of "table", but they are the same. When determining entities, think of them as nouns. In ER models, an entity is shown as a rounded rectangle, with its name on top and its attributes listed in the body of the entity shape.
Entity Attributes Also known as column, an attribute is a property or characteristic of the entity that holds it. An attribute has a name that describes the property and a type that describes the kind of attribute it is, such as varchar for a string, and int for integer.
The ER diagram example below shows an entity with some attributes in it. Primary Key Also known as PK, a primary key is a special kind of entity attribute that uniquely defines a record in a database table. In other words, there must not be two or more records that share the same value for the primary key attribute.
The ERD example below shows an entity 'Product' with a primary key attribute 'ID', and a preview of table records in database. Foreign Key Also known as FK, a foreign key is a reference to a primary key in table. It is used to identify the relationships between entities.
Note that foreign keys need not to be unique. Multiple records can share the same values. The ER Diagram example below shows an entity with some columns, among which a foreign key is used in referencing another entity.
Relationship A relationship between two entities signifies that the two entities are associated with each other somehow. For example, student might enroll into a course.
The entity Student is therefore related with Course, and the relationships is presented as a connector connecting between them. Cardinality Cardinality defines the possible number of occurrence in one entity which are associated to the number of occurrences in another. When present in an ERD, the entities Team and Player are inter-connected with a one-to-many relationship.
In an ER diagram, cardinality is represented as a crow's foot at the connector's ends. The three common cardinal relationships are one-to-one, one-to-many, and many-to-many. One-to-One cardinality example A one-to-one relationship is mostly used to split an entity in two to provide information concisely and make it more understandable.
The figure below shows an example of one-to-one relationship. One-to-Many cardinality example A one-to-many relationship refers to the relationship between two entities X and Y in which an instance of X may be linked to many instances of Y, but an instance of Y is linked to only one instance of X.
The figure below shows an example of one-to-many relationship.