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. Each course is taught by a single Professor. To maintain instruction quality, a Professor can deliver only one course Step 1 Entity Identification We have three entities Student Course Professor Step 2 Relationship Identification We have the following two relationships The student is assigned a course Professor delivers a course Step 3 Cardinality Identification For them problem statement we know that, A student can be assigned multiple courses A Professor can deliver only one course Step 4 Identify Attributes You need to study the files, forms, reports, data currently maintained by the organization to identify attributes.
Eliminate any redundant entities or relationships You need to make sure that all your entities and relationships are properly labeled There may be various valid approaches to an ER diagram. You need to make sure that the ER diagram supports all the data you need to store You should assure that each entity only appears a single time in the ER diagram Name every relationship, entity, and attribute are represented on your diagram Never connect relationships to each other You should use colors to highlight important portions of the ER diagram.
Report a Bug. Previous Prev. Next Continue. Home Testing Expand child menu Expand. SAP Expand child menu Expand. Web Expand child menu Expand. Must Learn Expand child menu Expand. Big Data Expand child menu Expand.
Live Project Expand child menu Expand. AI Expand child menu Expand. Toggle Menu Close. Search for: Search. 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. It is also called an atomic value. It is possible to break down composite attribute. This type of attribute does not include in the physical database. However, their values are derived from other attributes present in the database.
For example, age should not be stored directly. Instead, it should be derived from the DOB of that employee. Multivalued attributes can have more than one values. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity. An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values.
For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint. Entities are represented by means of their properties, called attributes.
All attributes have values. For example, a student entity may have name, class, and age as attributes. There exists a domain or range of values that can be assigned to attributes. For example, a student's name cannot be a numeric value. It has to be alphabetic. A student's age cannot be negative, etc. For example, a student's phone number is an atomic value of 10 digits.
Key is an attribute or collection of attributes that uniquely identifies an entity among entity set. An entity set may have more than one candidate key.
0コメント