Introduction to Entity-relationship modelling
Use meaningful names for relationship labels (ie avoid 'is a', 'has', etc.) on the ER diagram represents a notional entity and should therefore be singular. ( Unicode 25B2, 25BA, 25BC and 25C4) and insert them into your document. Under HIPAA and our contractual agreements with our HIPAA-covered entity which may cause our clients to terminate their relationship with us and may result . Division 4A—Relationship with the Privacy Act B Acts taken to be The main entities regulated by this Act are carriers and service providers.
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.Entity Relationship Diagram (ERD) Tutorial - Part 1
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.
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. 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.
Relationships between Entities in Entity Framework 6
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. 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. Sometimes this may not be possible for practical layout reasons - ie you may have to squeeze some tables to fit them on the page. Use elbow-style connectors This type of connectors ensures that your relationship lines are made up of horizontal and vertical sections. This will make it easier to follow a complex diagram. If straight connectors are used and the tables are not centre-aligned, the effect will be chaotic.
Avoid crossing lines Just common sense - lines that cross are more difficult to interpret. It is usually possible to avoid crossing lines by careful positioning of the tables. Differentiate relationship lines — ie no branching Each relationships line should be completely distinct - that is, no two lines should share a section.
Here is an example of what NOT to do: Align tables where possible This is another technique which simplifies the interpretation that the viewer has to do. Tables that are laid out in a regular grid are easier to understand.