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How is a relationship specified between two entities?
A relationship is an association that exists between two entities. For example, Instructor teaches Class or Student attends Class. Most relationships can also be stated inversely. The relationships on an Entity-Relationship Diagram are represented by lines drawn between the entities involved in the association.
Which concept allows us to represent relationship as entities?
Explanation: Diamond represents a relationship set and rectangle represents a entity set. Explanation: An entity set that has a primary key is termed a strong entity set.
How are relationships between entities represented in the relational data model?
Primary key and foreign key in the relation represents the relationship between entities in the relational data model. Primary key: In relational databases, a primary key is a key that is used for identifying and defining the characteristics uniquely of each row.
When do you use a mandatory relationship in a database?
We use mandatory relationships when we don’t want an occurrence of an entity to be store in the database unless it is related to an occurrence of the entity at the other end of the relationship. For example, if we didn’t want an engine in the database unless that engine was in a car, the end of the line next to the car entity would be ||.
How to mark a relationship as mandatory or optional?
I’ve got a tree structure where the relationship between the parent and the child can be marked as either mandatory, recommended, or optional. This structure can be up to 3 levels deep.
What does mandatory mean in an entity relationship?
Mandatory or optional roles indicate whether a certain role (with its associated entity set) is required for the relationship to exist. Total or partial participation indicate whether a certain relationship is required for an entity to exist.
How is optional participation used in a 1 : 1 relationship?
In a 1:1 relationship, if one entity has optional participation and the other has mandatory participation, the weak entity (the entity with optional participation) receives the key attribute from the mandatory entity to establish the link.