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    the degree of a relation is represented by the __________________ it contains, while the cardinality of a relation is represented by the ________________ it contains.

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    Chapter 8 The Entity Relationship Data Model – Database Design – 2nd Edition

    CHAPTER 8 THE ENTITY RELATIONSHIP DATA MODEL

    Adrienne Watt

    The entity relationship (ER) data model has existed for over 35 years. It is well suited to data modelling for use with databases because it is fairly abstract and is easy to discuss and explain. ER models are readily translated to relations. ER models, also called an ER schema, are represented by ER diagrams.

    ER modelling is based on two concepts:

    Entities, defined as tables that hold specific information (data)

    Relationships, defined as the associations or interactions between entities

    Here is an example of how these two concepts might be combined in an ER data model: Prof. Ba (entity) teaches (relationship) the Database Systems course (entity).

    For the rest of this chapter, we will use a sample database called the COMPANY database to illustrate the concepts of the ER model. This database contains information about employees, departments and projects. Important points to note include:

    There are several departments in the company. Each department has a unique identification, a name, location of the office and a particular employee who manages the department.

    A department controls a number of projects, each of which has a unique name, a unique number and a budget.

    Each employee has a name, identification number, address, salary and birthdate. An employee is assigned to one department but can join in several projects. We need to record the start date of the employee in each project. We also need to know the direct supervisor of each employee.

    We want to keep track of the dependents for each employee. Each dependent has a name, birthdate and relationship with the employee.

    ENTITY, ENTITY SET AND ENTITY TYPE

    An entity is an object in the real world with an independent existence that can be differentiated from other objects. An entity might be

    An object with physical existence (e.g., a lecturer, a student, a car)

    An object with conceptual existence (e.g., a course, a job, a position)

    Entities can be classified based on their strength. An entity is considered weak if its tables are existence dependent.

    That is, it cannot exist without a relationship with another entity

    Its primary key is derived from the primary key of the parent entity

    The Spouse table, in the COMPANY database, is a weak entity because its primary key is dependent on the Employee table. Without a corresponding employee record, the spouse record would not exist.

    An entity is considered strong if it can exist apart from all of its related entities.

    Kernels are strong entities.

    A table without a foreign key or a table that contains a foreign key that can contain nulls is a strong entity

    Another term to know is entity type which defines a collection of similar entities.

    An entity set is a collection of entities of an entity type at a particular point of time. In an entity relationship diagram (ERD), an entity type is represented by a name in a box. For example, in Figure 8.1, the entity type is EMPLOYEE.

    Figure 8.1. ERD with entity type EMPLOYEE.

    Existence dependency

    An entity’s existence is dependent on the existence of the related entity. It is existence-dependent if it has a mandatory foreign key (i.e., a foreign key attribute that cannot be null). For example, in the COMPANY database, a Spouse entity is existence -dependent on the Employee entity.

    KINDS OF ENTITIES

    You should also be familiar with different kinds of entities including independent entities, dependent entities and characteristic entities. These are described below.

    Independent entities

    Independent entities, also referred to as kernels, are the backbone of the database. They are what other tables are based on. Kernels have the following characteristics:

    They are the building blocks of a database.

    The primary key may be simple or composite.

    The primary key is not a foreign key.

    They do not depend on another entity for their existence.

    If we refer back to our COMPANY database, examples of an independent entity include the Customer table, Employee table or Product table.

    Dependent entities

    Dependent entities, also referred to as derived entities, depend on other tables for their meaning. These entities have the following characteristics:

    Dependent entities are used to connect two kernels together.

    They are said to be existence dependent on two or more tables.

    Many to many relationships become associative tables with at least two foreign keys.

    They may contain other attributes.

    The foreign key identifies each associated table.

    There are three options for the primary key:

    Use a composite of foreign keys of associated tables if unique

    Use a composite of foreign keys and a qualifying column

    Create a new simple primary key

    Characteristic entities

    Characteristic entities provide more information about another table. These entities have the following characteristics:

    They represent multivalued attributes.

    They describe other entities.

    They typically have a one to many relationship.

    The foreign key is used to further identify the characterized table.

    स्रोत : opentextbc.ca

    [Solved] The degree of a database relation is also called as:

    The correct answer is option 3. Concept: The degree of a relation (table) is the number of attributes (columns) in the given table. It is also called Ari

    Home DBMS Introduction and ER Model ER Model ER Diagrams

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    The degree of a database relation is also called as:

    This question was previously asked in

    UPPCL AE CS 2019 Official Paper (Held on 5 Nov 2019)

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    Extension Size Arity Cardinality

    Answer (Detailed Solution Below)

    Option 3 : Arity

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    Detailed Solution

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    The correct answer is option 3.

    Concept:

    The degree of a relation (table) is the number of attributes (columns) in the given table. It is also called Arity. An attribute is a property or feature of an entity.

    An entity can have an unlimited number of attributes. One of the attributes is considered to be the primary key. Attributes in an Entity-Relationship model are represented by an ellipse shape.

    Hence the correct answer is arity.Additional Information

    Cardinality refers to the relationships between the data in two database tables. Cardinality defines how many instances of one entity are related to instances of another entity.

    There are four types of cardinality.

    one to one relationship

    one to many relationship

    many to one relationship

    many to many relationship

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    More ER Model Questions

    Q1. Attributes in Entity­-Relationship diagram are represented by:­Q2. Which of the following components is not a part of Entity Relationship Diagrams?Q3. Let E1 and E2 be two entities in an E/R diagram with simple single-valued attributes. R1 and R2 are two relationships between E1 and E2, where R1 is one-to-many and R2 is many-to-many. R1 and R2 do not have any attributes of their own. What is the minimum number of tables required to represent this situation in the relational model?Q4. In ER-diagram, total participation of an entity into a relationship is shown using:Q5. The degree of a database relation is also called as:Q6. In RDBMS, domain constraint is also called:Q7. Which of the following statements is/are TRUE? (i) Weak entity has always total participation in the identifying relationship. (ii) Every relation schema must have at least one weak entity.Q8. Consider the "Supply" relationship relating the entities "Supplier" and "Product". Under default assumption. what will be the cardinality of the "Supply" relationship?Q9. Consider a relational table with a single record for each registered student with the following attributes. 1. Registration_Num: Unique registration number of each registered student 2. UID: Unique identity number, unique at the national level for each citizen 3. Bank Account_Num: Unique account number at the bank. A student can have multiple accounts or joint accounts. This attribute stores the primary account number. 4. Name: Name of the student 5. Hostel_Room: Room number of the hostel Which of the following options is INCORRECT?Q10. Given the basic ER and relational models, which of the following is INCORRECT?

    More Introduction and ER Model Questions

    Q1. Which type of storage device is a hard disk?Q2. Attributes in Entity­-Relationship diagram are represented by:­Q3. Which of the following components is not a part of Entity Relationship Diagrams?Q4. Let E1 and E2 be two entities in an E/R diagram with simple single-valued attributes. R1 and R2 are two relationships between E1 and E2, where R1 is one-to-many and R2 is many-to-many. R1 and R2 do not have any attributes of their own. What is the minimum number of tables required to represent this situation in the relational model?Q5. In ER-diagram, total participation of an entity into a relationship is shown using:

    स्रोत : testbook.com

    What is the degree of relation in DBMS?

    In this blog, we will learn about the degree of relationship in DBMS. We will also see what are its types depending upon various degree possible and discuss what is the maximum and minimum degree possible in relation.

    What is the degree of relation in DBMS?

    We have seen in our previous blog that there are three types of relationship. We have seen many examples in which two entity type like ‘Teacher’ and ‘Student’ are associated with a relationship. So, as we are talking about a relationship in DBMS, is it necessary that at least two entity sets should be there. What is the minimum number of entity sets required in a relationship? Can there be a relationship in which we have only one entity type? What is the maximum number of an entity type that can be a part of a DBMS relationship? We will answer all such question while we go through this blog. So, let's dive deeper and get started with the degree of relationship.

    Degree of Relationship

    The degree of a relationship is the number of entity types that participate(associate) in a relationship. By seeing an E-R diagram, we can simply tell the degree of a relationship i.e the number of an entity type that is connected to a relationship is the degree of that relationship.

    If we have two entity type ‘Customer’ and ‘Account’ and they are linked using the primary key and foreign key. We can say that the degree of relationship is 2 because here two entities are taking part in the relationship.

    Based on the number of entity types that are connected we have the following degree of relationships:

    Unary Binary Ternary N-ary

    Unary (degree 1)

    A unary relationship exists when both the participating entity type are the same. When such a relationship is present we say that the degree of relationship is 1.

    , Suppose in a classroom, we have many students who belong to a particular club-like dance club, basketball club etc. and some of them are club leads. So, a particular group of student is managed by their respective club lead. Here, the group is formed from students and also, the club leads are chosen from students. So, the ‘Student’ is the only entity participating here. We can represent this relationship using the E-R diagram as follows:

    So, here we get the answer to our first question which was asked in the introduction section. Yes, there can be only one entity type in a relationship and the minimum degree of a relationship can be one.

    Binary (degree 2)

    A binary relationship exists when exactly two entity type participates. When such a relationship is present we say that the degree is 2. This is the most common degree of relationship. It is easy to deal with such relationship as these can be easily converted into relational tables.

    We have two entity type ‘Customer’ and ‘Account’ where each ‘Customer’ has an ‘Account’ which stores the account details of the ‘Customer’. Since we have two entity types participating we call it a binary relationship. Also, one ‘Customer’ can have many ‘Account’ but each ‘Account’ should belong to only one ‘Customer’. We can say that it is a one-to-many binary relationship. (Learn more about types of relationships in DBMS from here)

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    Ternary(degree 3)

    A ternary relationship exists when exactly three entity type participates. When such a relationship is present we say that the degree is 3. As the number of entity increases in the relationship, it becomes complex to convert them into relational tables.

    We have three entity type ‘Employee’, ‘Department’ and ‘Location’. The relationship between these entities are defined as an employee works in a department, an employee works at a particular location. So, we can see we have three entities participating in a relationship so it is a ternary relationship. The degree of this relation is 3.

    N-ary (n degree)

    An N-ary relationship exists when ‘n’ number of entities are participating. So, any number of entities can participate in a relationship. There is no limitation to the maximum number of entities that can participate. But, relations with a higher degree are not common. This is because the conversion of higher degree relations to relational tables gets complex. We are making an E-R model because it can be easily be converted into any other model for implementing the database. But, this benefit is not available if we use higher degree relations. So, binary relations are more popular and widely used. Though we can make a relationship with any number of entity types but we don't do that.

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