Entity Relationship Mapping
The Entity Relationship Model At a basic level, databases store information about apply it in Entity Relationship Modeling Examples” for three sample databases. Attributes help distinguish one entity from other entities of the same type. One-to-One Relationship Definition - A one-to-one relationship in a relational database occurs when one parent record or field has either zero or one. In a one-to-one relationship, one record in a table is associated with one and only one record in another table. For example, in a school database, each student.
To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key.
One-to-many (data model) - Wikipedia
Each product entity would have name, price, and product ID attributes. The ER diagram representation of the product entity Representing Relationships Entities can participate in relationships with other entities.
For example, a customer can buy a product, a student can take a course, an artist can record an album, and so on. Like entities, relationships can have attributes: Our database could then record each sale and tell us, for example, that at 3: For example, each customer can buy any number of products, and each product can be bought by any number of customers. This is known as a many-to-many relationship. We can also have one-to-many relationships.
For example, one person can have several credit cards, but each credit card belongs to just one person. Looking at it the other way, a one-to-many relationship becomes a many-to-one relationship; for example, many credit cards belong to a single person.
Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine. We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively. The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship.
There are many relationships that may at first seem to be one-to-one, but turn out to be more complex. For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity.
Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is. In an ER diagram, we represent a relationship set with a named diamond. The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book.
The ER diagram representation of the customer and product entities, and the sale relationship between them.
One-to-one (data model)
Partial and Total Participation Relationships between entities can be optional or compulsory. In our example, we could decide that a person is considered to be a customer only if they have bought a product. On the other hand, we could say that a customer is a person whom we know about and whom we hope might buy something—that is, we can have people listed as customers in our database who never buy a product.
These are referred to as the participation constraints of the relationship. In an ER diagram, we indicate total participation with a double line between the entity box and the relationship diamond. From time to time, we encounter cases where we wonder whether an item should be an attribute or an entity on its own.
For example, an email address could be modeled as an entity in its own right.
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When in doubt, consider these rules of thumb: Is the item of direct interest to the database? Objects of direct interest should be entities, and information that describes them should be stored in attributes. Our inventory and sales database is really interested in customers, and not their email addresses, so the email address would be best modeled as an attribute of the customer entity.
Does the item have components of its own? If so, we must find a way of representing these components; a separate entity might be the best solution. In the student grades example at the start of the chapter, we stored the course name, year, and semester for each course that a student takes.
Can the object have multiple instances? If so, we must find a way to store data on each instance. The cleanest way to do this is to represent the object as a separate entity. In our sales example, we must ask whether customers are allowed to have more than one email address; if they are, we should model the email address as a separate entity.
Is the object often nonexistent or unknown? If so, it is effectively an attribute of only some of the entities, and it would be better to model it as a separate entity rather than as an attribute that is often empty. Consider a simple example: The ER diagram representation of student grades as a separate entity Entity or Relationship?
An easy way to decide whether an object should be an entity or a relationship is to map nouns in the requirements to entities, and to map the verbs to relations. All else being equal, try to keep the design simple, and avoid introducing trivial entities where possible; i. Intermediate Entities It is often possible to conceptually simplify many-to-many relationships by replacing the many-to-many relationship with a new intermediate entity sometimes called an associate entity and connecting the original entities through a many-to-one and a one-to-many relationship.
A passenger participates in an M: Thus, you have a many-to-many cardinality. The direction does not matter in this instance. You have the following cardinality: This cardinality is the most difficult to manage.
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Defining Relationships Here are the restrictions imposed on defining your relationships: You can define relationships only between CMP 2. You must declare both EJBs in the relationship within the same deployment descriptor.
One-to-one (data model) - Wikipedia
Each relationship can use only the local interface of the target EJB. The following are the requirements to define each cardinality type and its direction: The naming follows the same rules as for the persistence field abstract accessor methods.
For example, getAddress and setAddress methods are abstract accessor methods for retrieving and setting an address. Define each relationship--its cardinality and direction--in the deployment descriptor. Declare if you want the cascade delete option for the one-to-one, one-to-many, and many-to-one relationships.
The cascade delete is always specified on the "one" side of the relationship. In a relationship that sets or retrieves only a single entity, the object type passed back and forth must be the local interface of the target entity bean. In a relationship that sets or retrieves multiple objects, the object type passed back and forth is a Set or Collection containing local interface objects.
Example Definition of Abstract Accessor Methods for the Employee Example In this example, the employee can have only a single address, and you can retrieve the address only through the employee. This defines a one-to-one relationship that is unidirectional from the perspective of the employee.
Then the abstract accessor methods for the employee bean are as follows: The cardinality and direction of the relationship are defined in the deployment descriptor. Example Definition of One-To-Many Abstract Accessor Methods If the employee example included a one-to-many relationship, the abstract accessor methods would pass back and forth a Set or Collection of objects, each of which contains target bean local interface objects.
When you have a "many" relationship, multiple records are being passed back and forth. A department contains many employees. In this one-to-many example, the abstract accessor methods for the department retrieves multiple employees.