What is ER diagram in DBMS?
An ER diagram shows the relationship among entity sets. In terms of DBMS, an entity is a table or attribute of a table in database, so by showing relationship among tables and their attributes, ER diagram shows the complete logical structure of a database.
What is total participation in ER diagram?
Total Participation − Each entity is involved in the relationship. Total participation is represented by double lines. Partial participation − Not all entities are involved in the relationship.
What are the symbols used in ER diagram?
Entity Relationship Diagram Symbols & Notations mainly contains three basic symbols which are rectangle, oval and diamond to represent relationships between elements, entities and attributes. There are some sub-elements which are based on main elements in ERD Diagram.
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 is a weak entity in ER diagram?
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.
How are roles indicated in ER diagram?
The function that an entity plays in a relationship is called its role. Roles are normally explicit and not specified. They are useful when the meaning of a relationship set needs clarification. In the E-R diagram, this can be shown by labelling the lines connecting entities (rectangles) to relationships (diamonds).
What is a relationship called when an association is maintained between two entities?
Explanation: Ellipse represents attributes, rectangle represents entity. 6. What is a relationship called when it is maintained between two entities? Explanation: Primary key of one relation used as an attribute in another relation is called foreign key.
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.
Can relationships have attributes in ER diagram?
In ER model, entities have attributes which can be of various types like single-valued, multi-valued, composite, simple, stored, derived and complex. But relationships can also have attributes associated to them.
Can 2 entities have more than one relationship?
There can be more than one relationships between two entities. There should be only one class of employee If you have multiple categories of the employee then you can make child classes of Employee Class.
Can two entities have the same attributes?
1 Answer. It is ok to have tables with the same attributes. Just find sufficient meanings/predicates to record your application state.
Can a weak entity have a strong relationship?
A strong entity will always have a primary key. Strong entities are represented by a single rectangle. The relationship of two strong entities is represented by a single diamond….Difference between Strong and Weak Entity:
| S.NO | Strong Entity | Weak Entity |
|---|---|---|
| 3. | Strong entity is represented by single rectangle. | Weak entity is represented by double rectangle. |
What is weak entity set?
A weak entity set is an entity set that does not contain sufficient attributes to uniquely identify its entities. In other words, a primary key does not exist for a weak entity set. However, it contains a partial key called as a discriminator.
Can a weak entity have a primary key?
As the weak entities do not have any primary key, they cannot be identified on their own, so they depend on some other entity (known as owner entity). The weak entities have total participation constraint (existence dependency) in its identifying relationship with owner identity.
How can a weak entity set be strong?
Though weak entity set can be converted into strong entity set by simply adding appropriate attributes, this approach results in the redundant storage of primary key. The primary key of a weak entity set can be inferred from its relationship with the strong entity set.
What are the types of participation constraints?
There are two types of Participation constraint:
- Total participation.
- Partial participation.
What are the two types of participation constraints?
Participation constraint defines the least number of relationship instances in which an entity must participate. Total participation and Partial participation are the two types of participation constraints.
What do you mean by participation constraints?
In a Relationship, Participation constraint specifies the existence of an entity when it is related to another entity in a relationship type. This constraint specifies the number of instances of an entity that can participate in a relationship type.
What is cardinality ratio in ER diagram?
Description. Cardinality in DBMS defines the maximum number of relationship instances in which an entity can participate. Cardinality Ratios are- Many to Many Cardinality, Many to One Cardinality, One to Many Cardinality, One to One Cardinality. Author.