The most obvious one is that you go to see your doctor when you're ill (that's one kind of relationship between Patient and Physician). But you, as a Patient, may. Nurse and Doctor, rather than an entity; make Nurses and Doctors into one entity Note that the relationship between Patients and Nurses is M:M in the. Relationships - links between entities. • Example. • In a University database we might Entity/Relationship Diagrams .. A patient will be treated by one doctor.
Each patient has an identifier, the medical record number MRNand a name. Physician A member of the hospital medical staff who may admit patients to the hospital and who may administer medical treatments. Each physician has a physician ID and name. A patient's medical condition diagnosed by a physician.
Procedure codes are used for diagnostic and therapeutic procedures. Each order has an order ID, order date, and order time. In this manner, I was wondering if someone could look at this and derive the same ERD from the business rules I have or if I messed up the translation.
My considerations The specific issue I can not seem to wrap my head around is the existence of multiple relationships between the entity types. First of all, I do not even know what kind of cardinalities to put on regarding the relationship between Order and Treatment.
Another issue for me is the multiple relationships between Patient, Diagnosis and Physician. Also, the same issue with Physician, Patient, and Treament. Are these perhaps suppose to be ternary degree relationships, and or contain weak entity types to make this more understandable? For now just try to understand the overall process.
Suppose you are presented with the following situation and are told to create a database for it: Every department within our company is in only one division.
Introduction to Entity-relationship modelling
Each division has more than one department in it. For example, the New Business Developmentthe one managed by Mackenzieand Higher Education departments are both in the Marketing division.
This is a fairly clear description of a situation. Many things are left unsaid that we understand about the situation. The first step is to figure out the items of interest in this situation. In this document you will come across Problems. You should attempt to perform these before continuing the reading. Simply reading the problem and then reading the answer is not sufficientyou should attempt the problem yourself before you continue reading.
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.
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. 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.