What are the Advantages and disadvantages of Entity Relationship Model
The ER Model and ER Diagram. by K. Yue. 1. Some Disadvantages of ER diagrams: There are many ER diagram tools, with different relative merits. For our. Benefits Of The Entity Relationship Diagram Information Technology Essay. Basic idea of data model is a plan for building a database. Describing how can we. Apr 22, Advantages and Disadvantages of ER Model in DBMS. ER model is a logical representation of an enterprise data. ER model is a diagrammatic.
Diagrammatic notation associated with ER model. Consists of Entity, Attributes and Relationships. It is a thing in real world with its own independent existence.
Advantages and Disadvantages of ER Model in DBMS
Student, Faculty May be an object with physical or logical existence. 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.
- Entity Relationship Model in DBMS
- Entity Relationship Diagrams
- ER Diagrams And Its Benefits
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.
Advantages and Disadvantages of ER Model in DBMS
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. Self Describing Database — There should be dynamic online catalog based dictionary on relational model which keep information about tables data in database Comprehensive Data Sublanguage — The data access language SQL must be the only means of accessing data stored in the database and support DML, DDL etc.
Physical Data Independence — Any changes made in the way is physically stored must not affect applications that access data. Logical Data Independence — This rule states that changes to the database design should be done in a way without the users being aware of it. Integrity Independence — Data integrity constraints which are definable in the language must be stored in the database as data in table is, in the catalog and not in the application program. Non Subversion Rule — This rule states that there should be no bypass of constraints by any other languages.
Codd hence known as father of relational model. Was an attempt to simplify database structure by making use of tables and columns. Collection of 2-dimensional tables having unique names which consists of rows and columns.
Tables are known as relations columns are known as attributes and rows or records are known as tuples. A row in a table represents a relationship among a set of values. Thus a table represents a collection of relationships.
In a typical ER design, you can find symbols such as rounded rectangles and connectors with different styles of their ends that depict the entities, their attributes and inter-relationships. When to draw ER Diagrams? So, when do we draw ERDs? While ER models are mostly developed for designing relational database in terms of concept visualization and in terms of physical database design, there are still other situations when ER diagrams can help.
Here are some typical use cases. Database design - Depending on the scale of change, it can be risky to alter a database structure directly in a DBMS. To avoid ruining the data in a production database, it is important to plan out the changes carefully.
ERD is a tool that helps. By drawing ER diagrams to visualize database design ideas, you have a chance to identify the mistakes and design flaws, and to make correction before executing the changes in database. Database debugging - To debug database issues can be challenging, especially when the database contains many tables, which require writing complex SQL in getting the information you need. By visualizing a database schema with an ERD, you have a full picture of the entire database schema.
You can easily locate entities, view their attributes and to identify the relationships they have with others. All these allows you to analyze an existing database and to reveal database problem easier. Database creation and patching - ERD tool like Visual Paradigm supports database generation tool that can automate the database creation and patching process by means of ER diagrams.
So, with this ER Diagram tool your ER design is no longer just a static diagram but a mirror that reflects truly the physical database structure.
Aid in requirements gathering - Determine the requirements of an information system by drawing a conceptual ERD that depicts the high-level business objects of the system. 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.
ER Diagrams And Its Benefits
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.