Developing Entity Relationship Diagrams (ERDs) 

WHY:

Entity Relationship Diagrams are a major data modelling tool and will help organize the data in your project into entities and define the relationships between the entities. This process has proved to enable the analyst to produce a good database structure so that the data can be stored and retrieved in a most efficient manner. 

INFORMATION: 

Entity
A data entity is anything real or abstract about which we want to store data. Entity types fall into five classes: roles, events, locations, tangible things or concepts. E.g. employee, payment, campus, book. Specific examples of an entity are called instances. E.g. the employee John Jones, Mary Smith's payment, etc. 
Relationship
A data relationship is a natural association that exists between one or more entities. E.g. Employees process payments. Cardinality defines the number of occurrences of one entity for a single occurrence of the related entity. E.g. an employee may process many payments but might not process any payments depending on the nature of her job. 

Attribute
 

A data attribute is a characteristic common to all or most instances of a particular entity. Synonyms include property, data element, field. E.g. Name, address, Employee Number, pay rate are all attributes of the entity employee. An attribute or combination of attributes that uniquely identifies one and only one instance of an entity is called a primary key or identifier. E.g. Employee Number is a primary key for Employee. 

AN ENTITY RELATIONSHIP DIAGRAM METHODOLOGY: (One way of doing it)

1. Identify Entities 
Identify the roles, events, locations, tangible things or concepts about which the end-users want to store data. 
2. Find Relationships 
Find the natural associations between pairs of entities using a relationship matrix.
3. Draw Rough ERD 
Put entities in rectangles and relationships on line segments connecting the entities. 
4. Fill in Cardinality 
Determine the number of occurrences of one entity for a single occurrence of the related entity. 
5. Define Primary Keys 
Identify the data attribute(s) that uniquely identify one and only one occurrence of each entity. 
6. Draw Key-Based ERD 
Eliminate Many-to-Many relationships and include primary and foreign keys in each entity. 
7. Identify Attributes 
Name the information details (fields) which are essential to the system under development. 
8. Map Attributes 
For each attribute, match it with exactly one entity that it describes. 
9. Draw fully attributed ERD 
Adjust the ERD from step 6 to account for entities or relationships discovered in step 8. 
10. Check Results 
Does the final Entity Relationship Diagram accurately depict the system data? 

A SIMPLE EXAMPLE

A company has several departments. Each department has a supervisor and at least one employee. Employees must be assigned to at least one, but possibly more departments. At least one employee is assigned to a project, but an employee may be on vacation and not assigned to any projects. The important data fields are the names of the departments, projects, supervisors and employees, as well as the supervisor and employee number and a unique project number. 

1. Identify Entities 

The entities in this system are Department, Employee, Supervisor and Project. One is tempted to make Company an entity, but it is a false entity because it has only one instance in this problem. True entities must have more than one instance. 

2. Find Relationships 

We construct the following Entity Relationship Matrix: 
 
  Department Employee Supervisor     Project
Department   is assigned  run by   
Employee belongs to     works on
Supervisor runs      
Project   uses     

3. Draw Rough ERD 

We connect the entities whenever a relationship is shown in the entity Relationship Matrix. 

4. Fill in Cardinality 

From the description of the problem we see that: 

5. Define Primary Keys 

The primary keys are Department Name, Supervisor Number, Employee Number, Project Number. 

6. Draw Key-Based ERD 

There are two many-to-many relationships in the rough ERD above, between Department and Employee and between Employee and Project. Thus we need the associative entities Department-Employee and Employee-Project. The primary key for Department-Employee is the concatenated key Department Name and Employee Number. The primary key for Employee-Project is the concatenated key Employee Number and Project Number. 

7. Identify Attributes

The only attributes indicated are the names of the departments, projects, supervisors and employees, as well as the supervisor and employee NUMBER and a unique project number. 

8. Map Attributes 

Attribute   
 
Entity   
 
Attribute          
 
Entity
 
Department Name      
 
Department
 
Supervisor Number      
 
Supervisor
 
Employee Number        
 
Employee
 
Supervisor Name      
 
Supervisor
 
Employee Name        
 
Employee
 
Project Name         
 
Project
 
 
 
 
 
Project Number        
 
Project
 
          

9. Draw Fully Attributed ERD 

10. Check Results 

The final ERD appears to model the data in this system well. 

FURTHER DISCUSSION:

Step 1. Identify Entities 

A data entity is anything real or abstract about which we want to store data. Entity types fall into five classes: roles, events, locations, tangible things, or concepts. The best way to identify entities is to ask the system owners and users to identify things about which they would like to capture, store and produce information. Another source for identifying entities is to study the forms, files, and reports generated by the current system. E.g. a student registration form would refer to Student (a role), but also Course (an event), Instructor (a role), Advisor (a role), Room (a location), etc. 

Step 2. Find Relationships 

There are natural associations between pairs of entities. Listing the entities down the left column and across the top of a table, we can form a relationship matrix by filling in an active verb at the intersection of two entities which are related. Each row and column should have at least one relationship listed or else the entity associated with that row or column does not interact with the rest of the system. In this case, you should question whether it makes sense to include that entity in the system.  
. A student is enrolled in one or more courses
     subject    verb           objects

Step 3. Draw Rough ERD 

Using rectangles for entities and lines for relationships, we can draw an Entity Relationship Diagram (ERD). 

Step 4. Fill in Cardinality 

At each end of each connector joining rectangles, we need to place a symbol indicating the minimum and maximum number of instances of the adjacent rectangle there are for one instance of the rectangle at the other end of the relationship line. The placement of these numbers is often confusing. The first symbol is either 0 to indicate that it is possible for no instances of the entity joining the connector to be related to a given instance of the entity on the other side of the relationship, 1 if at least one instance is necessary or it is omitted if more than one instance is required. For example, more than one student must be enrolled in a course for it to run, but it is possible for no students to have a particular instructor (if they are on leave). 

The second symbol gives the maximum number of instances of the entity joining the connector for each instance of the entity on the other side of the relationship. If there is only one such instance, this symbol is 1. If more than 1, the symbol is a crows foot opening towards the rectangle. 

If you read it like a sentence, the first entity is the subject, the relationship is the verb, the cardinality after the relationship tells how many direct objects (second entity) there are.  

     I.e. A student is enrolled in one or more courses
              subject    verb           objects

Step 5. Define Primary Keys 

For each entity we must find a unique primary key so that instances of that entity can be distinguished from one another. Often a single field or property is a primary key (e.g. a Student ID). Other times the identifier is a set of fields or attributes (e.g. a course needs a department identifier, a course number, and often a section number; a Room needs a Building Name and a Room Number). When the entity is written with all its attributes, the primary key is underlined. 

Step 6. Draw Key-Based ERD 

Looking at the Rough Draft ERD, we may see some relationships which are non-specific or many-to-many. I.e., there are crows feet on both ends of the relationship line. Such relationships spell trouble later when we try to implement the related entities as data stores or data files, since each record will need an indefinite number of fields to maintain the many-to-many relationship. 

Fortunately, by introducing an extra entity, called an associative entity for each many-to-many relationship, we can solve this problem. The new associative entity's name will be the hyphenation of the names of the two originating entities. It will have a concatenated key consisting of the keys of these two entities. It will have a 1-1 relationship with each of its parent entities and each parent will have the same relationship with the associative entity that they had with each other before we introduced the associative entity. The original relationship between the parents will be deleted from the diagram. 

The key-based ERD has no many-to-many relationships and each entity has its primary and foreign keys listed below the entity name in its rectangle. 

Step 7. Identify Attributes 

A data attribute is a characteristic common to all or most instances of a particular entity. In this step we try to identify and name all the attributes essential to the system we are studying without trying to match them to particular entities. The best way to do this is to study the forms, files and reports currently kept by the users of the system and circle each data item on the paper copy. Cross out those which will not be transferred to the new system, extraneous items such as signatures, and constant information which is the same for all instances of the form (e.g. your company name and address). The remaining circled items should represent the attributes you need. You should always verify these with your system users. (Sometimes forms or reports are out of date.) 

Step 8. Map Attributes 

For each attribute we need to match it with exactly one entity. Often it seems like an attribute should go with more than one entity (e.g. Name). In this case you need to add a modifier to the attribute name to make it unique (e.g. Customer Name, Employee Name, etc.) or determine which entity an attribute "best' describes. If you have attributes left over without corresponding entities, you may have missed an entity and its corresponding relationships. Identify these missed entities and add them to the relationship matrix now. 

Step 9. Draw Fully-Attributed ERD 

If you introduced new entities and attributes in step 8, you need to redraw the entity relationship diagram. When you do so, try to rearrange it so no lines cross by putting the entities with the most relationships in the middle. If you use a tool like Systems Architect, redrawing the diagram is relatively easy. 

Even if you have no new entities to add to the Key-Based ERD, you still need to add the attributes to the Non-Key Data section of each rectangle. Adding these attributes automatically puts them in the repository, so when we use the entity to design the new system, all its attributes will be available. 

Step 10. Check Results 

Look at your diagram from the point of view of a system owner or user. Is everything clear? Check through the Cardinality pairs. Also, look over the list of attributes associated with each entity to see if anything has been omitted.