ERD of a company database — Images were taken from Fundamentals An entity relationship model, also called an entity-relationship (ER) diagram, is a graphical representation of entities (which Recursive Relationship. pugliablog.info an ER diagram for car insurance company. Relationship Unary( Recursive) relationship The number of associated entity is one. The entity relationship (ER) data model has existed for over 35 years. In an entity relationship diagram (ERD), an entity type is represented by a name in a box. An example of a multivalued attribute from the COMPANY database, as seen in . For some entities in a unary relationship, a separate column can be created.
A table can have any number of rows within physical storage limitations. Each row is a record.
We could delete all of the rows and the table would still exist. The shorthand notation to express the structure of a table is: We underline the primary key. A table can have an unlimited number of foreign keys linking to other tables. We use a dashed underline for foreign keys. Properties of Tables Relations: Each table in a database has a unique name.
No multivalued fields are allowed in a table. Each row is unique. Each column has a unique field name. The sequence of rows as well as columns is insignificant and can be interchanged without changing the data. Merge tables with the same primary key based on the existing database schema for the firm - the system you are currently designing will probably add to an existing schema.
In transforming entities into tables, recall there are three types of entities: 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.
model - ER-Diagram: Ternary Relationship - How to read properly? - Stack Overflow
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?Attributes to relationships
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.
- Unary Relationships (recursive)
- Fundamentals of Database Management Systems, Second Edition by MARK L. GILLENSON
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.
DATA MODELING USING THE ENTITY-RELATIONSHIP MODEL
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.
Unary Relationships (recursive) | Open Textbooks for Hong Kong
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. 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? 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.