What is Entity Relationship Diagram? Webopedia Definition
An entity-relationship (ER) diagram is a graphical representation of entities and In an ER diagram, symbols are commonly used to to represent the types of. A relationship type is illustrated in an ERD using a diamond symbol. The "n" represents an "arbitrary number of instances", and the "1" represents "at most one . Because it is easier to examine structures graphically than to describe them in a ER models are normally represented in an entity relationship diagram (ERD) or data model components were introduced, three types of relationships among.
The solution is to either adjust the model or the SQL. This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue. The second issue is a 'chasm trap'.
A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. For example, a Building has one-or-more Rooms, that hold zero-or-more Computers.
One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list. Another relation between Building and Computers is needed to capture all the computers in the building. This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model.
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.
Introduction to Entity-relationship modelling
For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification. Understanding these problems are integral to understanding the text.
The answer to the problem appears in the text immediately after the problem. What are the items of interest here? It seems here that the situation is concerned with divisions, departments, and employees or managers.
It gives some details about which contains which, how they are related to each other, and provides some examples of each, but basically the situation is concerned with these three entity types. An entity type is a collection of entities that share a common definition. An entity is a person, place, concept, or thing about which the business needs data. So, Department is the name of one entity type.
One instance of this entity type is the New Business Development department. The Marketing division is an instance of the Division entity type. Mackenzie is one instance of the Employee entity type. Instances of entity types are referred to as entities. You can touch an entity but an entity type is simply an idea.
Person is an idea entity type while Scott, Nancy, Lindsey, and Mackenzie are touchable entities. Entity types provide us with a means for making generalisations about entities. The Higher Education department is in one division. But we know more than the facts about each individual department being in one division.
We know that all new departments will also be in just one division. And if there is a new division, it, too, will have departments that are unique to the division. So, instead of providing information in the form of statements about specific entities, we use a more powerful and concise format and provide information in the form of statements about relationships among entity types.
Thus, in ER modelling we look for relationships among entity types because it is easier and more concise to speak of relationships among general entity types rather than the touchable entities themselves. A municipal bond from Detroit Ford Clothes Employee The municipal bond is an entity; bond is a possible entity type. Ford is an entity; manufacturer is a possible entity type.
Clothes could be either: Employee is an entity type; Angela and Natalie are example entities. Back to our example: From the description we can assume that there are more entities for each entity type. Go back and read the situation description if you do not think this is immediately obvious. From the description there is some sort of relationship between Department and Division and another sort of relationship between Department and Employee. The first relationship is one of containment: On the other hand, each team one ball can only be in one bucket a conference.
In this instance the bucket is the division and the balls are the departments. The second relationship tells us that an employee has a certain relationship relative to a certain Department, namely, that the employee manages the department. Determining the relationships among entity types is another important step in the process of ER modelling. A relationship is an association between entity types. What would you name these two relationships? The defining characteristic of a relationship is that several entity types are involved.
So something like a name or birth date would not be a relationship since only one entity is involved. Now we have identified three entity types Employee, Department, Division and two relationships among these entity types manages, contains. Now we can begin to represent the problem in the language of ER modelling. ER models are usually represented graphically. The language we are going to use represents entity types as rectangles and relationships as diamonds. Below is the representation of the situation we are working with.
Notice that the contains relationship is drawn between the two entities that it is associated with.
Similarly for the manages relationship. This simplified ER model tells us that: Division is related to department through a relationship called contains. Departments are related to employees through a relationship called manages. Employees are not directly related to divisions.
Certainly we know more about the problem than this. Consider the relationship between divisions and departments. We know that divisions have multiple departments and departments can only be contained within one division.
entity-relationship diagram (model)
Or, for every one division there can be many departments. In the language of ER modelling this is called a 1: What is the relationship between departments and managers? Fill in the blanks with either a one or a many: The relationship between department and a managing employee is different.
Certainly you can imagine an instance in which a department has co-managers. That possibility is just as viable as the possibility I have assumed. This is part of the attraction of this type of work. If you were actually creating a database in this example, you would have to ask someone what the situation actually is. But since you are just given this description, you have to come up with some assumption.
In other words, for every one department there can be, at most, one managing employee. This information can also be represented in the ER diagram: As you might have determined, the M part of a relationship is represented by putting an M next to the appropriate entity type in the relationship while the 1 part is represented by a 1.
The ER diagram now represents much more information than it did above: Any one division can contain many departments. Any one department can be contained in, at most, one division. Any department can have, at most, one managing employee or manager. Any manager can manage, at most, one department.
If you are a bit confused about all this 1: Several other questions remain about this situation that are not addressed in the description: What is the minimum number of departments in a division?
Does a department have to be associated with a division?
Does a department have to have a manager? These questions would have to be answered before we complete the ER model. And we will answer these questions later. For now we are going to stop this part of the analysis since the purpose of this example is to demonstrate what ER modelling is all about. The ER modelling process is not something for which a set of steps can be given and then performed.
Entity Relationship Model in DBMS
The process contains almost as much art as science. Some steps are performed many times and many decisions are re-visited and revised. Given these conditions, a broad outline can be given: Determine what entity types are involved. Determine which entity types are related.
Refine the definition of the relationships. Understand now that there are several methods for representing ER models graphically.
Notice what has happened with this situation. Initially we had a text description of the problem.
- Entity Relationship Model in DBMS
- Entity-relationship modelling
After analysing it and making some necessary assumptions, we created an ER diagram that reflects the situation accurately and makes explicit the relationship among the entity types. This is why we perform ER modelling. It is quite a straight-forward step to go from this ER model to an implemented database.
Remember why we are doing all this: 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.