The Entity Relationship Model - Learning MySQL [Book]
An entity–relationship model (ER model for short) describes interrelated things of interest in a form as boxes (entities) that are connected by lines (relationships) which express the associations and dependencies between entities. An ER. CS Home Data Modeling ERD Entities Relationships Attributes Example posavski-obzor.info Software Lab Relationships are the associations between the entities. Check out this guide for a comprehensive look at all ER diagram symbols and notation. Relationships are associations between or among entities.
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.
- Entity–relationship model
- entity-relationship diagram (model)
- Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data. 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. Additional techniques can be used for other aspects of systems. For instance, ER models roughly correspond to just 1 of the 14 different modeling techniques offered by UML.
Even where it is suitable in principle, ER modeling is rarely used as a separate activity.
One reason for this is today's abundance of tools to support diagramming and other design support directly on relational database management systems. Let's see how each relationship association is being managed by entity framework.
A student can have only one or zero addresses. Entity framework adds the Student reference navigation property into the StudentAddress entity and the StudentAddress navigation entity into the Student entity.
This means that Standard can have many Teachers whereas Teacher can associate with only one Standard. To represent this, the Standard entity has the collection navigation property Teachers please notice that it's pluralwhich indicates that one Standard can have a collection of Teachers many teachers. And the Teacher entity has a Standard navigation property reference propertywhich indicates that Teacher is associated with one Standard.
This makes it a One-to-Many relationship.
An attribute may be formed from smaller parts; for example, a postal address is composed of a street number, city, ZIP code, and country. Some attributes can have multiple values for a given entity. For example, a customer could provide several telephone numbers, so the telephone number attribute is multivalued. Attributes help distinguish one entity from other entities of the same type. We could use the name attribute to distinguish between customers, but this could be an inadequate solution because several customers could have identical names.
To be able to tell them apart, we need an attribute or a minimal combination of attributes guaranteed to be unique to each individual customer. The identifying attribute or attributes form a key.
In our example, we can assume that no two customers have the same email address, so the email address can be the key. However, we need to think carefully about the implications of our choices.
For example, if we decide to identify customers by their email address, it would be hard to allow a customer to have multiple email addresses.
Any applications we build to use this database might treat each email address as a separate person, and it might be hard to adapt everything to allow people to have multiple email addresses. 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. 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.