A Entity Relationship Diagram showing optional and mandatory relation example . You can edit this (12). E-R Diagram for Hotel management system. (10). In a mandatory relationship, every instance of one In an optional relationship, any instance of. The participation condition defines whether it is mandatory or optional for an the overlapping specialization/generalization relationship in the ER diagram.
Mandatory and optional roles: In a relationship, roles can be optional or mandatory.
This affects whether a relationship instance can exist without an entity in a given role. Mandatory roles are indicated with a solid association line, optional roles are indicated with a dotted line.
Roles aren't often talked about in database tutorials, but they're an important concept. Consider a marriage - a relationship with two mandatory roles filled by the same entity set. In most relationships, the entity sets also define the roles, but when an entity set appears multiple times in a single relationship, we distinguish them in different roles. In the example above, a Patient can Purchase Medicine with or without a Prescription.
A Purchase can't exist without a Patient and Medicine, but a Prescription is optional overall, though it may be required in specific cases. A weak entity is an entity that can't be identified by its own attributes and therefore has another entity's key as part of its own. An identifying relationship is the relationship between a weak entity and its parent entity. Both the identifying relationship and the weak entity are indicated with double borders.
Weak entity sets must necessarily participate totally in their identifying relationship. In this example, a Prescription contains LineItems which are identified by the Prescription's key and a line number. For examples of non-identifying relationships, see the previous examples.
Introduction to Entity-relationship modelling
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.How to draw ER diagram
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.
Mandatory and optional Participation | Entity-relationship Diagrams in Database Design
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.
optional and mandatory relation example ( Entity Relationship Diagram)
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.
However, an instance of the order entity must be related to one and only one customer. The 'zero' option is not available to a weak entity. The relationship between an instance of the order entity and an instance of the customer entity is therefore a mandatory relationship. Mandatory Relationship An order cannot exist without a customer, therefore The relationship between an 1 instance of the order entity and 2 an instance of the customer entity is a mandatory relationship.
Identifying weak entities and their associated mandatory relationships Identifying weak entities and their associated mandatory relationships can be very important for maintaining the consistency and integrity of the database. Consider the effect, for example, of storing an order without knowing the customer to which it belongs.
There would be no way to ship the item to the customer, and the company would lose business. By the same token, we typically specify the relationship between an order and the order lines the specific items on the order as mandatory because we do not want to allow an order line to exist in the database without it being related to an order. An order line is meaningless without knowing the order to which it belongs.