The entity-relationship (ER) model is one of the most popular database design for creating query compilers which are more productive, effective and forceful. It is also the case that relationship names (role names, actually—half a but from your author's experience with the Barker/Ellis entity/relationship approach. The sentences are easier and more forceful to read, though, than would be the. The entity relationship (ER) data model has existed for over 35 years. It is well suited to data modelling for use with databases because it is fairly abstract and is .
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". 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.
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. These tools can readily extract database diagrams that are very close to ER diagrams from existing databases, and they provide alternative views on the information contained in such diagrams.
In a survey, Brodie and Liu  could not find a single instance of entity—relationship modeling inside a sample of ten Fortune companies.
Badia and Lemire  blame this lack of use on the lack of guidance but also on the lack of benefits, such as lack of support for data integration. The enhanced entity—relationship model EER modeling introduces several concepts not in ER modeling, but are closely related to object-oriented design, like is-a relationships.
We are finding out all we need to know to create a database that will hold our data. And a well-defined database can be a very useful tool for solving business problemsand it is also in high demand by recruiters.
You will learn how to perform the steps necessary to create such a database in later chapters. In this section I present more detail on some of the basic concepts. In the example in an earlier section, we saw that divisions are directly associated with departments and departments are directly associated with employees. No direct association between division and employee was given. This does not mean that there is no relationship between division and employee.
In fact, the ER diagram tells us that there is a relationship between the two: Given any one division, there can be many employees managing departments within that division.
Certainly, this is not earth shattering news. But it is in the ER diagram. The above fact is not represented as a separate relationship between division and employee because it can be inferred from existing relationships. An ER diagram should contain the minimum number of relationships necessary to reflect the situation.
For relationships between two entity types, there are three basic cardinalities. Each of the following descriptions are given in terms of a relationship between entity type X and entity type Y. One entity of type Y can be associated with, at most, one entity of type X. A car has only one steering wheel and a steering wheel can only be installed in one car. M one-to-many One entity of type X can be associated with many entities of type Y.
A building can have many rooms but a room can be in, at most, one building. M many-to-many One entity of type X can be associated with many entities of type Y.
One entity of type Y can be associated with many entities of type X. A car can have many options and an option can be installed on many cars.
Determine the cardinality of the relationships between the following four pairs of entity types. For each relationship you have to answer two questions: Answering these two questions gives you the answer to the following questions.
For example, if you answered M to the first question and 1 for the second question, then this relationship between entity types X and Y is of cardinality M: Patient under care of primary care physician Physician performs operation Doctors have speciality in disease Needle injected into patient It would seem that at any particular time a patient can only have one primary care physician and that any physician can have many patients M: One physician can perform many operations and one operation can be performed by many physicians M: One doctor can have specialities in many diseases and one disease can be the speciality of many doctors M: In this section we examine the minimum number of entities in a relationship.
Existence is given as optional, mandatory, or unknown. This is best clarified with an example. Consider again the example discussed in Section 2. Specifically, focus on the manage relationship between department and employee.
We know the cardinality is 1: This tells us that at most one department is managed by an employee and an employee can manage, at most, one department. Be sure you understand the distinction between these two phrases.
- Entity–relationship model
The existence of this relationship tells us the fewest number of departments that can be managed by an employee and the fewest number of employees that can manage a department. Only one of the following can be true: Similarly, only one of the following may be true: For each set of three above, which ones would you choose?
It is not entirely clear from the situation description which of the above are true. I make the relatively standard assumptions that a department must have at least one manager and that an employee need not be the manager of any department. Thus, the existence of this relationship is mandatory in one direction and optional in the other. Going back to the definition of existence, we can also look at this situation in this way: Given any randomly chosen department, there must be an employee on the other side of the manage relationship.
Thus, the relationship is mandatory in this direction. Given any randomly chosen employee, there need not be any department on the other side of the manage relationship. Thus, the relationship is optional in this direction. I assume that the contains relationship is mandatory in both directions. Given this information, the ER diagram is modified in the following manner: This diagram is beginning to look a little complicated but remember the following pieces of information and it gets a little easier: The marks on the lines tell you the minimum number in a relationship.
A dash on the line looks like a 1; it tells you the minimum number in the relationship is one so the existence is mandatory. A circle on the line looks line a 0; it tells you the minimum number in the relationship is zero so the existence is optional.
If there are not any marks on the line, then the existence is unknown. The marks below the line tell you the maximum number in a relationship. An M tells you the cardinality is many while a 1 tells you it is one. Look at the manages relationship again. Cover up the line connecting the manages diamond with the department rectangle.
Now all you see are the department entity type, the employee entity type, and the right side of the line with a dash on it and a one below it. This tells us that a department is managed by at least the dash one employee and at most the 1 one employee.
A department is managed by one and only one employee. Cover up the line connecting the manages diamond with the employee rectangle.
Now all you see are the department entity type, the employee entity type, and the left side of the line with a circle on it and a 1 below it. This tells us that an employee does not have to manage any departments the circle and may manage at most the 1 one department.
An employee may manage no more than one department. For each of the relationships listed in Problem 5: Define the existence in both directions. Draw the ER diagram for the relationship. Write out two sentences that represent what the ER diagram says. If it is not important, it should not be in the database. In an accounting database you would expect to find entity types for expenses, assets, liabilities, expenditures, deposits, etc.
You would not expect to find entity types for colour of check, quality of dollar bills received, etc. The database is supposed to reflect realitybut only the part of reality that is important to the company.
This allows us to make generalisations about that type. This is a powerful capability; however, sometimes we want to make a generalisation only about a certain subset of those entities and another generalisation about the rest of the entities. Consider a simple example. Suppose you have an accounting database which keeps track of accounts receivable and accounts payable. Of course the database keeps track of the companies to which you owe money and the companies that owe you money.
For all these companies, you keep track of their mailing address and a contact person. For the companies that owe you money you keep track of how much they owe you. For the companies that you owe money you keep track of how much you owe them.
Should we have three entity types: That would be a mess. That is why the concept of entity subtypes was created. In this company example, what is the entity type?
Entity–relationship model - Wikipedia
What are the subtypes? In this database you should define a company entity type with two subtypes: The company entity type stores all facts that are common attributesin this case, the address and contact person. There are many situations in which subtypes can be created but should not be. Only create subtypes if the subtype is involved in relationships that the other subtypes are not or if the subtype needs to have additional facts stored with it.
If one of these two requirements is not met, then do not create the subtype. What is the relationship among the following? List a few facts common to all items for each question.
List a few facts about each subtype that is not common to the other subtype. Also draw the ER diagram for each. Consider the following figure. Many students would first suggest the diagram on the rightdivide customers into investors and attendees and show that investors buy stocks and attendees register for seminars.
I suggest that the figure on the left is better. What is it that makes an investor an investor? And what is it that makes an attendee an attendee? She registers for seminars. Is there anything about an investor that keeps her from being an attendee? Do you want to prevent investors from being classified as attendees or vice versa?
So, define relationships buy and register for the customer entity. Investors can be listed by choosing only those customers that are in the buy relationship. Attendees can be listed by choosing only those customers that are in the register relationship.
Thus, if a relationship defines the members of a proposed subtype, then use the relationship instead of the subtype. What we are trying to discern here is the difference between a type of a thing and an actual thing.
This is a pretty easy concept when comparing people and Joe. People is the type and Joe is the instance. The section is an actual class that meets at an actual time with an actual teacher and actual students.Entity Relationship Diagram (ERD) Tutorial - Part 2
CT is a type of thing that is an idea that only becomes real when you come into contact with one of its instances e. Realize that this is a different distinction than that between entity types and entities. In this example, CT is one specific instance of the entity type Course and section 2 of CT in Winter is one specific instance of the entity type Section.
Thus, both are entities and neither one is an entity type. Boeing is a type of plane and a specific Boeing that flies through the air with passengers in it is an instance of this type. Boeing is a specific instance of the entity type plane type and a flying Boeing with passengers is a specific instance of an entity type plane.
In this section I hope to make these ideas a little more clear. Attributes are the characteristics of an entity type that we are interested in.
An attribute is a descriptor whose values are associated with individual entities of a specific type. The attribute value for any single entity can have only one value at a given time. This value can change over time. An attribute of an employee might be salary.