Question

Consider a marriage entity type and attributes marriage date, marriage location, husband, wife. Each marriage will only have one value for each of these attributes. Illustrate the marriage entity and its single-valued attributes in an ERD .

Short answer

The ERD for this exercise will have a rectangle representing 'Marriage' entity. Four ovals representing 'Marriage Date', 'Marriage Location', 'Husband' and 'Wife', each connected to the 'Marriage' entity by straight lines.

Step-by-step solution

Identify the Entity

In this problem, the entity to be considered for the Entity-Relationship Diagram (ERD) is 'marriage'.

Identify the Attributes

The attributes for the 'marriage' entity identified in this problem are 'marriage date', 'marriage location', 'husband',and 'wife'.

Construct the ERD

The 'marriage' entity would be represented as a rectangle in the ERD. The attributes 'marriage date', 'marriage location', 'husband', and 'wife' would then be represented as ovals and linked to the 'marriage' entity rectangle through straight lines. This can be done using any diagram creation tool or hand drawing.

Key concepts

Single-Valued Attribute

In an Entity-Relationship Diagram (ERD), a **single-valued attribute** is one that holds only one value for each instance of an entity. This means that for any given entity instance, such as a specific marriage in our example, each attribute would have just one set value. For instance, the "marriage date" attribute is single-valued because any specific marriage can occur on only one day. Similarly, "marriage location," "husband," and "wife" are all single-valued, as each attribute can have only one unique value associated with each instance of the marriage entity. Key things to remember about single-valued attributes are:

• They differ from multivalued attributes, which can hold multiple values for a single entity instance.
• They simplify the data structure by ensuring each record of an entity has unique, specific attributes.
• They help in maintaining consistency and integrity within the ER model.

Understanding single-valued attributes is crucial when designing databases, as they ensure clarity and prevent data duplication.

Entity Type

In the world of database design, an **entity type** is a main object or concept within an Entity-Relationship Model (ER Model) that can be distinctly identified and can have attributes related to it. In our example, the entity type is 'marriage.' This entity type represents an occurrence or instance in the database that has certain characteristics, like a marriage date, location, husband, and wife. Each entity of the 'marriage' entity type can be viewed as a record or row of data in a database table:

• An entity type is usually represented by a rectangle in an ER diagram.
• Each entity type can have multiple attributes, which define its properties or qualities.
• Entities of the same type lack inherent meaning without their attribute values.

Understanding the entity type is fundamental because it helps outline what data the database needs to store and how these pieces relate to each other.

ER Model

The **ER Model** is a method used in database design to visually represent the structure of a database. It outlines how entities relate to one another, providing a blueprint for constructing and understanding database systems. Key features of an ER Model include:

• Entities: The major objects or concepts represented in an ER diagram—like the 'marriage' entity.
• Attributes: Characteristics or properties that define an entity, like marriage date and location.'
• Relationships: Connections between entities, indicating how data is related.

In an ERD for the 'marriage' entity, you'd see:

• A rectangle labeled as 'marriage' to depict the main entity.
• Ovals connecting to this rectangle, each labeled with attributes such as "marriage date," "marriage location," "husband," and "wife."
• Lines connecting attributes to the entity, indicating their direct relationship.

An ER Model is integral to crafting well-structured databases, as it translates complex database concepts into easily understood visuals. This helps developers and stakeholders discuss and refine database requirements efficiently.