What does dotted line mean in SQL?
The dotted/dashed line means, that SQL Server does not enforce referential integrity for this relationship.
What is double line in ER diagram?
A double line indicates the total participation constraint in an ER model (see Figure 17). However, the participation of student in LeaderOf in Figure 10 is partial, because a student might be a team leader.
How do you read ER diagram symbols?
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.
What is strong and weak relationship in ER diagram?
A strong relation arises between types of entities in cases where the existence of one type of entity is impossible without another type of entity. A weak relation arises between two types of entities in the case when there is no direct relationship between them.
Which attribute in an ER diagram is underlined with dashed dotted line?
The key attribute of the weak entity is represented by dashed underline.
What do double lines indicate?
A dashed yellow line indicates that passing is allowed. White lines separate lanes for which travel is in the same direction. A double white line indicates that lane changes are prohibited.
What does a solid line mean in a ERD?
relationship is weakA dashed line means that the relationship is strong, whereas a solid line means that the relationship is weak.
What is Isa in ER diagram?
The ISA (pronounced as "is-a") relationship is most frequently used to represent generalization and specialization in an ER Diagram. It is used in situations when an entity type contains certain entities that has special properties not shared by all the entities.
What is foreign key in ER diagram?
Foreign keys are not used in E-R models, but they are used in relational databases to indicate an attribute that is the primary key of another table. Foreign keys are used to establish a relationship when both tables have the same attribute [5].
What is difference between weak and strong entity?
Strong entity is independent of other entities. Weak entity is dependent on strong entity. A strong entity is represented by single rectangle. A weak entity is represented by double rectangle.
What is partial key?
Partial Key : The set of attributes that are used to uniquely identify a weak entity set is called the Partial key. Only a bunch of the tuples can be identified using the partial keys. The partial Key of the weak entity set is also known as a discriminator.
What is the difference between entity and weak entity?
A weak entity is dependent on a strong entity to ensure its existence. Unlike a strong entity, a weak entity does not have any primary key. It instead has a partial discriminator key. A weak entity is represented by a double rectangle.
What is a multivalued attribute in ER diagram?
Multivalued attribute: An attribute that can hold multiple values is known as multivalued attribute. It is represented with double ovals in an ER Diagram. For example – A person can have more than one phone numbers so the phone number attribute is multivalued.
What are constraints in ER diagram?
Constraints are used for modeling limitations on the relations between entities. Mapping cardinality or cardinality ratio. Participation constraints.
What is weak entity in ER diagram?
A weak entity is a type of entity which doesn't have its key attribute. It can be identified uniquely by considering the primary key of another entity. For that, weak entity sets need to have participation. In above ER Diagram examples, “Trans No” is a discriminator within a group of transactions in an ATM.
What is foreign key in ER diagram?
Foreign keys are not used in E-R models, but they are used in relational databases to indicate an attribute that is the primary key of another table. Foreign keys are used to establish a relationship when both tables have the same attribute [5].
What is an ER model?
Creating an entity-relationship (ER) model is to visually represent the structure of a business database, where data equates to entities (or objects) that are linked by defined relationships expressing dependencies and requirements. By nature it is an abstract visualization, the first step in the design process towards creating a logical and functional database.
What is an entity relationship diagram?
Entity-Relationship Diagram (ERD) serves for detailed description of structures and databases. An ERD represents a diagram made up mainly of rectangular blocks (for entities, or data) linked with relationships. The links between the blocks describe relations between these entities. There are three types of relationships: one-to-one,
What is Chen's ERD?
The Chen's ERD notation is still used and is considered to present a more detailed way of representing entities and relationships. To create an ERD, software engineers mainly turn to dedicated drawing software, which contain the full notation resources for their specific database design - ERD symbols and meanings.
What is Crow's foot diagram?
Crow's foot diagrams represent entities as boxes, and relationships as lines between the boxes. Different shapes at the ends of these lines represent the cardinality of the relationship.
Can you draw an ERD diagram on a Mac?
Drawing ER diagrams on a Mac is smooth and easy when you have a software tool well-suited for professional ERD drawing. ConceptDraw DIAGRAM software for Mac extended with the Entity-Relationship Diagram (ERD) solution is the best tool for drawing ER diagrams on a Mac.
Entity
Entities are represented by means of rectangles. Rectangles are named with the entity set they represent.
Attributes
Attributes are the properties of entities. Attributes are represented by means of ellipses. Every ellipse represents one attribute and is directly connected to its entity (rectangle).
Relationship
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 is key in ER diagram?
Keys are one way to categorize attributes. ER diagrams help users to model their databases by using various tables that ensure that the database is organized, efficient, and fast. Keys are used to link various tables in a database to each other in the most efficient way possible.
What is ordinality in a line?
Ordinality, on the other hand, is the minimum number of times an instance in one entity can be associated with an instance in the related entity. Cardinality and ordinality are shown by the styling of a line and its endpoint, according to the chosen notation style.
What are ERD attributes?
ERD attributes are characteristics of the entity that help users to better understand the database. Attributes are included to include details of the various entities that are highlighted in a conceptual ER diagram.
What are the types of entities in a relationship diagram?
Entities are typically nouns such as product, customer, location, or promotion . There are three types of entities commonly used in entity relationship diagrams. These shapes are independent from other entities, and are often called parent entities, since they will often have weak entities that depend on them.
What is a relationship diagram?
Within entity-relationship diagrams, relationships are used to document the interaction between two entities. Relationships are usually verbs such as assign, associate, or track and provide useful information that could not be discerned with just the entity types.
What is the difference between cardinality and ordinality?
Cardinality refers to the maximum number of times an instance in one entity can relate to instances of another entity. Ordinality, on the other hand, is the minimum number of times an instance in one entity can be associated with an instance in the related entity.
What does a dashed line mean in a class diagram?
A dashed line means that the relationship is strong, whereas a solid line means that the relationship is weak. On the following diagram how do we decide that the relationship between the Room and Class entities is strong.
Why do we draw a dashed line?
Consider a weak but not ID-dependent relationship; We draw a dashed line because it is a weak relationship. Usually occurs utilizing a composite key for primary key, which means one of this composite key components must be the primary key of the parent entity.
