The Entity Relationship Model - Learning MySQL [Book]
In the ER diagram, an entity set is represented by a rectangle containing the entity Some attributes can have multiple values for a given entity. . Figure 5. E/R Diagrams. ◇In an entity-relationship diagram: 6. Example: ◇Entity set Beers has two attributes, name and ◇In a many-many relationship, an entity. In an entity relationship diagram (ERD), an entity type is represented by a name in a box. Many to many relationships become associative tables with at least two foreign keys. They may contain .. Student table for question 6, by A. Watt.
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. Partial and Total Participation Relationships between entities can be optional or compulsory.
Entity Relationship Diagram
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. 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?
- Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
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.
Entity Relationship Diagram
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: Any given flight can have many passengers with a booking. Any given passenger can have bookings on many flights. Hence, we can consider the many-to-many relationship to be in fact two one-to-many relationships, one each way. This points us to the existence of a hidden intermediate entity, the booking, between the flight and the passenger entities.
The requirement could be better worded as: The intermediate booking entity between the passenger and flight entities Each passenger can be involved in multiple bookings, but each booking belongs to a single passenger, so the cardinality of this relationship is 1: Similarly, there can be many bookings for a given flight, but each booking is for a single flight, so this relationship also has cardinality 1: Since each booking must be associated with a particular passenger and flight, the booking entity participates totally in the relationships with these entities.
We described partial and total participation earlier in Partial and Total Participation. For example, we generally call family members by only their first name or nickname.
Developing an Application
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What is more, over types of diagrams are supported. Offers free technology support and lifelong upgrade chance. The different entity relationship diagram is using the different symbols to represent the relationships between entities in a database for most information management projects.
But ER diagrams often use boxes to represent entities. Diamonds are normally used to represent relationships and ovals are used to symbolize attributes.
Entity Relationship Symbols Entities Symbol: Entity instance-person, place, object, event, concept Relationships Symbol: Chen ERD uses rectangles to represent entities, and diamonds to represent relationships appropriate for first-class objects. If an entity set participates in a relationship set, they are connected with a line. Attributes are drawn as ovals and are connected with a line to exactly one entity or relationship set.