In order to begin constructing the basic model, the modeler must analyze the information gathered during the requirements analysis for the purpose of:
- classifying data objects as either entities or attributes,
- identifying and defining relationships between entities,
- naming and defining identified entities, attributes, and relationships,
- documenting this information in the data document.
To accomplish these goals the modeler must analyze narratives from users, notes from meeting, policy and procedure documents, and, if lucky, design documents from the current information system.
Although it is easy to define the basic constructs of the ER model, it is not an easy task to distinguish their roles in building the data model. What makes an object an entity or attribute? For example, given the statement “employees work on projects”. Should employees be classified as an entity or attribute? Very often, the correct answer depends upon the requirements of the database. In some cases, employee would be an entity, in some it would be an attribute.
While the definitions of the constructs in the ER Model are simple, the model does not address the fundamental issue of how to identify them. Some commonly given guidelines are:
- entities contain descriptive information
- attributes either identify or describe entities
- relationships are associations between entities
There are various definitions of an entity:
- “Any distinguishable person, place, thing, event, or concept, about which information is kept” [BRUC92]
- “A thing which can be distinctly identified” [CHEN76]
- “Any distinguishable object that is to be represented in a database” [DATE86]
- “…anything about which we store information (e.g. supplier, machine tool, employee, utility pole, airline seat, etc.). For each entity type, certain attributes are stored”. [MART89]
These definitions contain common themes about entities:
- an entity is a “thing”, “concept” or, object”. However, entities can sometimes represent the relationships between two or more objects. This type of entity is known as an associative entity.
- entities are objects which contain descriptive information. If an data object you have identified is described by other objects, then it is an entity. If there is no descriptive information associated with the item, it is not an entity. Whether or not a data object is an entity may depend upon the organization or activity being modeled.
- an entity represents many things which share properties. They are not single things. For example, King Lear and Hamlet are both plays which share common attributes such as name, author, and cast of characters. The entity describing these things would be PLAY, with King Lear and Hamlet being instances of the entity.
- entities which share common properties are candidates for being converted to generalization hierarchies(See below)
- entities should not be used to distinguish between time periods. For example, the entities 1st Quarter Profits, 2nd Quarter Profits, etc. should be collapsed into a single entity called Profits. An attribute specifying the time period would be used to categorize by time.
- not every thing the users want to collect information about will be an entity. A complex concept may require more than one entity to represent it. Others “things” users think important may not be entities.
Attributes are data objects that either identify or describe entities. Attributes that identify entities are called key attributes. Attributes that describe an entity are called non-key attributes.
The process for identifying attributes is similar except now you want to look for and extract those names that appear to be descriptive noun phrases.
Attribute values should be atomic, that is, present a single fact. Having disaggregated data allows simpler programming, greater reusability of data, and easier implementation of changes. Normalization also depends upon the “single fact” rule being followed. Common types of violations include:
- simple aggregation – a common example is Person Name which concatenates first name, middle initial, and last name. Another is Address which concatenates, street address, city, and zip code. When dealing with such attributes, you need to find out if there are good reasons for decomposing them. For example, do the end-users want to use the person’s first name in a form letter? Do they want to sort by zip code?
- complex codes – these are attributes whose values are codes composed of concatenated pieces of information. An example is the code attached to automobiles and trucks. The code represents over 10 different pieces of information about the vehicle. Unless part of an industry standard, these codes have no meaning to the end user. They are very difficult to process and update.
- text blocks – these are free-form text fields. While they have a legitimate use, an over reliance on them may indicate that some data requirements are not met by the model.
- mixed domains – this is where a value of an attribute can have different meaning under different conditions.
Derived attributes and code values
Two areas where data modeling experts disagree is whether derived attributes and attributes whose values are codes should be permitted in the data model.
Derived attributes are those created by a formula or by a summary operation on other attributes. Arguments against including derived data are based on the premise that derived data should not be stored in a database and therefore should not be included in the data model. The arguments in favor are:
- derived data is often important to both managers and users and therefore should be included in the data model.
- it is just as important, perhaps more so, to document derived attributes just as you would other attributes
- including derived attributes in the data model does not imply how they will be implemented.
A coded value uses one or more letters or numbers to represent a fact. For example, the value Gender might use the letters “M” and “F” as values rather than “Male” and “Female”. Those who are against this practice cite that codes have no intuitive meaning to the end-users and add complexity to processing data. Those in favor argue that many organizations have a long history of using coded attributes, that codes save space, and improve flexibility in that values can be easily added or modified by means of look-up tables.
Relationships are associations between entities. Typically, a relationship is indicated by a verb connecting two or more entities. For example:
employees are assigned to projects
As relationships are identified they should be classified in terms of cardinality, optionality, direction, and dependence. As a result of defining the relationships, some relationships may be dropped and new relationships added.
Cardinality quantifies the relationships between entities by measuring how many instances of one entity are related to a single instance of another. To determine the cardinality, assume the existence of an instance of one of the entities. Then determine how many specific instances of the second entity could be related to the first. Repeat this analysis reversing the entities. For example,
employees may be assigned to no more than three projects at a time; every project has at least two employees assigned to it.
Here the cardinality of the relationship from employees to projects is three; from projects to employees, the cardinality is two. Therefore, this relationship can be classified as a many-to-many relationship.
If a relationship can have a cardinality of zero, it is an optional relationship.If it must have a cardinality of at least one, the relationship is mandatory. Optional relationships are typically indicated by the conditional tense. For example,
an employee may be assigned to a project.
Mandatory relationships, on the other hand, are indicated by words such as must have. For example,
a student must register for at least three course each semester.
In the case of the specific relationship form (1:1 and 1:M), there is always a parent entity and a child entity. In one-to-many relationships, the parent is always the entity with the cardinality of one. In one-to-many relationships, the choice of the parent entity must be made in the context of the business being modeled. If a decision cannot be made, the choice is arbitrary.