Question

Draw an ERD for marriages between two people. For persons include birthdate, first name, last name, and a unique person id. Consider marriage to be a relationship between two people and suppose we want our model to allow for people to have more than one marriage. Use the date of the marriage as a discriminator.

Short answer

The ERD should contain two entities: 'Person' and 'Marriage'. Properties of 'Person' entity are person id, first name, last name, and birthdate. 'Marriage' entity has marriage date as a property. The relationship is a 'Many-to-Many' relationship between 'Person' and 'Marriage', discriminated by the date of marriage.

Step-by-step solution

Identify the Entities

In this problem, the entities are the 'Person' and 'Marriage'. The 'Person' entity will be represented by an oval shape, enclosing the entity's name.

Identify the Attributes of Each Entity

Attributes for the 'Person' entity include the person id (a unique identifier), first name, last name, and birthdate. Attributes for the 'Marriage' entity is the marriage date. A rectangle is bounded around each attribute, and these rectangles are connected to their respective entity using a straight line.

Identify the Relationships

There is a relationship entity identified in the problem statement, defined as a marriage between two 'Person' entities. This relationship is represented with a diamond shape connected to both 'Person' entities. Since a 'Person' can have more than one 'Marriage', this indicates a 'Many-to-Many' (M:N) relationship between 'Person' and 'Marriage'.

Implement Discriminator

The marriage date acts as a discriminator here, differentiating multiple marriages. This can be shown in the ERD diagram as a role attached to the relationship entity.

Key concepts

Database Design

Database design is a critical process in creating a database system that effectively manages data. It involves defining the structure of the database, including the organization of data, the relationships among data elements, and the constraints imposed on the database. A well-designed database should ensure data consistency, integrity, and optimum performance.

In the context of an ERD for marriages, database design begins with understanding the requirements: What information do we need to store about individuals and their marriages? The answer leads to the creation of entities, setting up attributes for these entities, and defining the relationships between them. Database design is an iterative process, where normalization often plays a role in eliminating redundancies to enhance data integrity and efficiency.

ERD Entities

In an Entity-Relationship Diagram (ERD), an entity represents a real-world object or concept that has an independent existence. Entities are typically depicted as rectangles or ovals in the diagram.

In our marriage ERD example, the main entity is 'Person,' which would represent individuals who can enter into marriages. Critical to the ERD's clarity, each entity must have attributes that describe its properties. For 'Person,' these attributes include personal identification details such as birthdate, first name, and last name. The unique identifier for each person, such as a person ID, ensures that each record is distinguishable from another, imperative for accurately modeling and querying the database.

ERD Relationships

Relationships in an ERD illustrate how entities associate with each other within the database. They are depicted by lines connecting entities, often with a diamond shape signifying the nature of the relationship.

In our exercise, a 'Marriage' is depicted as a relationship between two 'Person' entities. This relationship can have its attributes, such as the marriage date, which gives specific context to the association. The cardinality of the relationship dictates how one entity relates to another in terms of numbers; one-to-one, one-to-many, or many-to-many. In the provided exercise, it is crucial to note that one person can have multiple marriages over time, showcasing the need for a 'Many-to-Many' relationship.

Many-to-Many Relationship

A many-to-many relationship occurs when multiple records in one entity can relate to multiple records in another. In an ERD, we resolve a many-to-many relationship using an associative entity that enables us to break down this complex relationship into two one-to-many relationships.

The marriage ERD illustrates a many-to-many relationship as people can have multiple marriages, and each marriage can involve two different people. To model this in a database, typically, a join table is created, which includes foreign keys referencing the primary keys of the associated entities. This way, one can track and distinguish each unique pairing of persons and marriages.

Unique Identifiers in Databases

Unique identifiers, or primary keys, are crucial elements in database design. They ensure that each record within a table can be uniquely identified and referenced. These identifiers are necessary for preventing ambiguity and ensuring the integrity of relationships within the database.

In our scenario of the marriage ERD, every person is given a unique person ID. This attribute guarantees that even two persons with the same name and birthdate can be differentiated. Unique identifiers are often an incremental number, a UUID (Universally Unique Identifier), or a combination of multiple fields to establish uniqueness. They also play a pivotal role in establishing relationships by acting as foreign keys in related tables.