Table of Contents
Can an entity have two relationships with another entity?
5 Answers. There is no reason you can’t have multiple relationships between the same two entities, your understanding is wrong in this respect.
What is an entity that does not require a relationship to another entity for identification?
In a relational database, a weak entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to.
Can an entity have a relationship with itself?
It is possible for an entity to have a relationship with itself; for example, an entity Staff could have a relationship with itself, as one member of staff could supervise other staff. This is known as a recursive or involute relationship, and would be represented in an entity-relationship diagram as shown below.
Is the number of entity classes that participate in a relationship in an ER model?
E) An identifier in the E-R model is equivalent to a key in the relational model. C) The value of an identifier may identify a set of entity instances. The number of entity classes involved in a relationship is known as the relationship’s: A) maximum cardinality.
Are identifiers that consist of two or more attributes?
Composite identifiers consist of two or more attributes. When designing a database, first identify the entities, then determine the attributes, and finally establish the relationships.
What is identifier in ERD?
Primary Key* or identifier is an attribute or a set of attributes that uniquely identifies an instance of the entity. For example, for a student entity, student number is the primary key since no two students have the same student number. We can have only one primary key in a table.
Can a relationship have 3 entities?
A ternary relationship is an association among three entities. This type of relationship is required when binary relationships are not sufficient to accurately describe the semantics of the association.
What entity indicates that its minimum cardinality is zero?
Can lines cross in an ERD?
3 Answers. Yes, because it can impede understanding of the graph/diagram. lines crossing (you might have seen in some diagrams a small bridge when lines are crossing) number of line bends.
How do I make an ERD?
Below points show how to go about creating an ER diagram.
- Identify all the entities in the system. An entity should appear only once in a particular diagram.
- Identify relationships between entities. Connect them using a line and add a diamond in the middle describing the relationship.
- Add attributes for entities.
What is the purpose of creating ERD?
An entity-relationship diagram (ERD) is crucial to creating a good database design. It is used as a high-level logical data model, which is useful in developing a conceptual design for databases. An entity is a real-world item or concept that exists on its own.
How do you write an ERD?
Here are some best practice tips for constructing an ERD:
- Identify the entities. The first step in making an ERD is to identify all of the entities you will use.
- Identify relationships. Look at two entities, are they related?
- Describe the relationship. How are the entities related?
- Add attributes.
- Complete the diagram.
What is a weak entity in ER diagram?
ER Diagram: Weak Entity Weak entity is an entity that depends on another entity. Weak entity doesn’t have anay key attribute of its own. Double rectangle is used to represent a weak entity.
What is ER model explain with example?
Entity Relationship Model (ER Modeling) is a graphical approach to database design. It is a high-level data model that defines data elements and their relationship for a specified software system. For example, each employee of an organization is a separate entity.
How do you read an ERD diagram?
You read the diagrams first from left to right and then from right to left. In the case of the name-address relationship in following figure, you read the relationships as follows: names can be associated with zero or exactly one address; addresses can be associated with zero, one, or many names.
How primary key is defined in ER diagram?
The primary key is a candidate key that is selected by the database designer to be used as an identifying mechanism for the whole entity set. It must uniquely identify tuples in a table and not be null. The primary key is indicated in the ER model by underlining the attribute.
Can a relationship have a primary key?
Columns that define primary keys in one table in a relational model can have a relationship with columns in one or more other tables. Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key.
What does double rectangles show in entity relationship diagram?
An entity that cannot be uniquely identified by its own attributes and relies on the relationship with other entity is called weak entity. The weak entity is represented by a double rectangle.
What is the difference between the strong entity set and weak entity set?
A strong entity is an entity that is independent of any other entity in a schema. It has sufficient attributes to form a primary key. A weak entity is an entity set that cannot be uniquely identified by its attributes alone. It does not have sufficient attributes to form a primary key.
How foreign key is represent in ER diagram?
The ER model does not use foreign keys to represent relationships. It uses lines between boxes. The lines have some kind of indicator for cardinality at either end or both ends. Sometimes, a relationship will be indicated separately by a diamond.
What do double diamonds represent in an ER diagram?
Explanation: Diamonds represent relationship sets in an ER diagram. Relationship sets define how two entity sets are related in a database. Explanation: The double diamonds represent the relationship sets linked to weak entity sets. Weak entity sets are the sets that do not have a primary key.
What does Diamond mean in ER diagram?
To quote Wikipedia: The UML representation of a composition relationship shows composition as a filled diamond shape on the containing class end of the lines that connect contained class(es) to the containing class. In other words, a patient can have any number of account s associated with (possessed by) it.
What is diamond shape in ER diagram?
Relationships are represented by diamond-shaped box. 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.
What are diamonds in ER diagram?
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.
What are ER methods?
ERM is a database modeling technique that generates an abstract diagram or visual representation of a system’s data that can be helpful in designing a relational database. These diagrams are known as entity-relationship diagrams, ER diagrams or ERDs.
What does cardinality mean?
Cardinality means two things in databases. In this sense, cardinality means whether a relationship is one-to-one, many-to-one, or many-to-many. So you’re really talking about the relationship cardinality. Cardinality’s official, non-database dictionary definition is mathematical: the number of values in a set.
Is a relationship in ER diagram?
In ER diagram, relationship type is represented by a diamond and connecting the entities with lines. A set of relationships of same type is known as relationship set.
What are the 3 types of relationships in a database?
There are three types of relationships between the data you are likely to encounter at this stage in the design: one-to-one, one-to-many, and many-to-many. To be able to identify these relationships, you need to examine the data and have an understanding of what business rules apply to the data and tables.
What is the full form of ER diagram?
An Entity Relationship (ER) Diagram is a type of flowchart that illustrates how entities such as people, objects or concepts relate to each other within a system.
Can a relationship have an attribute?
Like entities, relationships can have attributes: we can define a sale to be a relationship between a customer entity (identified by the unique email address) and a given number of the product entity (identified by the unique product ID) that exists at a particular date and time (the timestamp).