Key concepts: Entity, attribute, and entity type
An entity–relationship model (ER model for short) describes interrelated things of interest in a A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between Diagrams created to represent attributes as well as entities and relationships may be called . Feb 1, The following is an example of a definition for an employee entity: entity type ( fundamental, attributive, associative, subtype/supertype), This attribute formalizes the relationship Employee is Assigned to Department.". The Entity Relationship Model At a basic level, databases store information about Attributes help distinguish one entity from other entities of the same type. an artificial attribute that's defined to be unique and can therefore be used as a.
Clearly, there may be several possible keys that could be used to identify an entity; we choose one of the alternative, or candidate, keys to be our main, or primary, key. You usually make this choice based on how confident you are that the attribute will be non-empty and unique for each individual entity, and on how small the key is shorter keys are faster to maintain and use.
Attributes comprising the primary key are shown underlined. The parts of any composite attributes are drawn connected to the oval of the composite attribute, and multivalued attributes are shown as double-lined ovals. Similarly, a product price could be a positive rational number. Attributes can be empty; for example, some customers may not provide their telephone numbers.
You should think carefully when classifying an attribute as multivalued: The sales database requirements may specify that a product has a name and a price. To distinguish between products, we can assign a unique product ID number to each item we stock; this would be the primary key.
ER Model Basic Concepts
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.
- Key concepts: Entity, attribute, and entity type
- Entity–relationship model
- Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
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.
In our example, we could decide that a person is considered to be a customer only if they have bought a product. See Entity-Relationship Modelling 2 for details. Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model".
ER Model - Basic Concepts
It is an intensional model. At the latest since Carnapit is well known that: The first part comprises the embedding of a concept in the world of concepts as a whole, i. The second part establishes the referential meaning of the concept, i. Extension model[ edit ] An extensional model is one that maps to the elements of a particular methodology or technology, and is thus a "platform specific model".
The UML specification explicitly states that associations in class models are extensional and this is in fact self-evident by considering the extensive array of additional "adornments" provided by the specification over and above those provided by any of the prior candidate "semantic modelling languages". It incorporates some of the important semantic information about the real world. Plato himself associates knowledge with the apprehension of unchanging Forms The forms, according to Socrates, are roughly speaking archetypes or abstract representations of the many types of things, and properties and their relationships to one another.
Limitations[ edit ] ER assume information content that can readily be represented in a relational database. They describe only a relational structure for this information. They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data.
entity-relationship diagram (model)
For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification. An alternative is to model change separately, using a process modeling technique. We use such models to address the question What will the data in our system be about?
Entities are the things or objects in our system. They represent conceptual categories, not specific objects. In a store there may be a shop assistant called Eve. When she quits and another person called James is hired, he has the same role as Eve: Moreover, the store can also hire other shop assistants.
In our system, we will only have the general entity ShopAssistant. This entity is the role that both Eve and James have.
ShopAssistant is the entity in our system. In a shop you have different products: The ring and the earrings are instances of the same concept — in this case, the Product. So our store system should have an entity known as Product.
Entities are like nouns. They can be almost anything, depending on the system. A car or a customer could be entities for an auto repair shop; student, teacher and class can be entities for a school data model.
Patient, doctor, nurse, and room can be entities for a medical data model. As you can see, the list of entities is the vocabulary of a given system. In this series, you are going to see lots of different entities. Naming Your Entities Entities are represented with rectangles.
Database Modeling Course (2): Entities and Attributes
Inside each rectangle, we put the name of the entity: Car, Person, Doctor, etc. Here are the entities from a car identification system: This is particularly important for huge data models with dozens of entities.
In this series of articles, entities are always going to be uppercase Student or Nurse instead of student or nursebut other data models may use a different system. Keep in mind, though, that short and descriptive entity names are always good form. Filling Out the Details with Attributes Each entity is associated with some details known as attributes. Remember shop assistants Eve and James?