You need to study the files, forms, reports, data currently maintained by the organization to identify attributes. You can also conduct interviews with various stakeholders to identify entities. Initially, it's important to identify the attributes without mapping them to a particular entity. Once, you have a list of Attributes, you need to map them to the identified entities.
Ensure an attribute is to be paired with exactly one entity. If you think an attribute should belong to more than one entity, use a modifier to make it unique. For the sake of ease we have considered just one attribute. What is ER Diagrams? Here, are prime reasons for using the ER Diagram Helps you to define terms related to entity relationship modeling Provide a preview of how all your tables should connect, what fields are going to be on each table Helps to describe entities, attributes, relationships ER diagrams are translatable into relational tables which allows you to build databases quickly ER diagrams can be used by database designers as a blueprint for implementing data in specific software applications The database designer gains a better understanding of the information to be contained in the database with the help of ERP diagram ERD is allowed you to communicate with the logical structure of the database to users Components of the ER Diagram This model is based on three basic concepts: Entities Attributes Relationships Example For example, in a University database, we might have entities for Students, Courses, and Lecturers.
Example of Entities: A university may have some departments. Relationship Relationship is nothing but an association among two or more entities. For example: You are attending this lecture I am giving the lecture Just loke entities, we can classify relationships according to relationship-types: A student attends a lecture A lecturer is giving a lecture.
Weak Entities A weak entity is a type of entity which doesn't have its key attribute. It does not have enough attributes to build a primary key. It is represented by a rectangle symbol. It is represented by a double rectangle symbol. It contains a Primary key represented by the underline symbol.
It contains a Partial Key which is represented by a dashed underline symbol. The member of a strong entity set is called as dominant entity set.
The member of a weak entity set called as a subordinate entity set. Primary Key is one of its attributes which helps to identify its member. In a weak entity set, it is a combination of primary key and partial key of the strong entity set.
In the ER diagram the relationship between two strong entity set shown by using a diamond symbol. The relationship between one strong and a weak entity set shown by using the double diamond symbol. The connecting line of the strong entity set with the relationship is single.
The line connecting the weak entity set for identifying relationship is double. Attributes It is a single-valued property of either an entity-type or a relationship-type. For example, a lecture might have attributes: time, date, duration, place, etc. An attribute is represented by an Ellipse Types of Attributes Description Simple attribute Simple attributes can't be divided any further.
For example, a student's contact number. It is also called an atomic value. Composite attribute It is possible to break down composite attribute.
For example, a student's full name may be further divided into first name, second name, and last name. Derived attribute This type of attribute does not include in the physical database. Name of the relationship is written inside the diamond-box.
All the entities rectangles participating in a relationship, are connected to it by a line. A relationship where two entities are participating is called a binary relationship. Cardinality is the number of instance of an entity from a relation that can be associated with the relation. The following image reflects that only one instance of each entity should be associated with the relationship. It depicts one-to-one relationship.
The following image reflects that only one instance of entity on the left and more than one instance of an entity on the right can be associated with the relationship.
It depicts one-to-many relationship. The following image reflects that more than one instance of an entity on the left and only one instance of an entity on the right can be associated with the relationship. A lecturer from the specific department takes each course, and each lecturer teaches a various group of students. Relationship is nothing but an association among two or more entities. Entities take part in relationships. We can often identify relationships with verbs or verb phrases.
It can be identified uniquely by considering the primary key of another entity. For that, weak entity sets need to have participation. One entity from entity set X can be associated with at most one entity of entity set Y and vice versa.
Example: One student can register for numerous courses. However, all those courses have a single line back to that one student. One entity from entity set X can be associated with multiple entities of entity set Y, but an entity from entity set Y can be associated with at least one entity. More than one entity from entity set X can be associated with at most one entity of entity set Y. However, an entity from entity set Y may or may not be associated with more than one entity from entity set X.
For example, Students as a group are associated with multiple faculty members, and faculty members can be associated with multiple students. Following are the steps to create an ER Diagram:.
You need to study the files, forms, reports, data currently maintained by the organization to identify attributes. You can also conduct interviews with various stakeholders to identify entities. Once, you have a list of Attributes, you need to map them to the identified entities. Ensure an attribute is to be paired with exactly one entity.
If you think an attribute should belong to more than one entity, use a modifier to make it unique. Once the mapping is done, identify the primary Keys. If a unique key is not readily available, create one. For the sake of ease we have considered just one attribute. Skip to content. What is ER Diagram? Entity Relationship Diagram Example. What is ER Model? Following are the main components and its symbols in ER Diagrams: Rectangles: This Entity Relationship Diagram symbol represents entity types Ellipses : Symbol represent attributes Diamonds: This symbol represents relationship types Lines: It links attributes to entity types and entity types with other relationship types Primary key: attributes are underlined Double Ellipses: Represent multi-valued attributes ER Diagram Symbols.
A student must be assigned to at least one or more Courses.
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