Introduction to Entity-relationship modelling
Significant overlap between E-R model and relational model. – Biggest the relation. – Specify any foreign key references to other relations . One-to-many mapping from manager to worker. – “Many” side N-ary Relationship Primary Keys. An ER diagram can be mapped into a relational schema. 4 An n-ary relationship set R relates n entity sets E1 En; each Does every department have a manager? . Hospital: It maintains all patients visited, including age and address. An entity type within ER diagram is turned into a table. You may This is the same for N − ary relationship of ER diagrams. For instance, The.
Persons personidname, lastname, email Phones phoneidpersonid, phone personid within the table Phones is a foreign key referring to the personid of Persons 3. For instance, let us consider the case where the Person has or optionally has one wife. You can place the primary key of the wife within the table of the Persons which we call in this case Foreign key as shown below. Persons personidname, lastname, emailwifeid Wife wifeidname Or vice versa to put the personid as a foreign key within the Wife table as shown below: Persons personidname, lastname, email Wife wifeidnamepersonid For cases when the Person is not married i.
N Relationships This is the tricky part! For simplicity, use attributes in the same way as 1: For instance, the Person can have a House from zero to manybut a House can have only one Person. To represent such relationship the personid as the Parent node must be placed within the Child table as a foreign key but not the other way around as shown next: It should convert to: Persons personidname, lastname, email House houseidnumaddress, personid 5.
N Relationships We normally use tables to express such type of relationship. When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. 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. 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.
Entity–relationship model - Wikipedia
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.
How to Convert ER Diagram to Relational Database | Learn Databases
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.