Entity-Relationship Diagrams
Entity-Relationship Diagrams (ERD)
Data models are tools used in analysis to describe the data requirements and assumptions in the system from a top-down perspective. They also set the stage for the design of databases later on in the SDLC.
There are three basic elements in ER models:
Entities are the "things" about which we seek information.
Attributes are the data we collect about the entities.
Relationships provide the structure needed to draw information from multiple entities.
Generally, ERD's look like this: adapted from another professor.
Developing an ERD
Developing an ERD requires an understanding of the system and its components. Before discussing the procedure, let's look at a narrative created byProfessor Harman.
Consider a hospital:
Patients are treated in a single ward by the doctors assigned to them. Usually each patient will be assigned a single doctor, but in rare cases they will have two.
Heathcare assistants also attend to the patients, a number of these are associated with each ward.
Initially the system will be concerned solely with drug treatment. Each patient is required to take a variety of drugs a certain number of times per day and for varying lengths of time.
The system must record details concerning patient treatment and staff payment. Some staff are paid part time and doctors and care assistants work varying amounts of overtime at varying rates (subject to grade).
The system will also need to track what treatments are required for which patients and when and it should be capable of calculating the cost of treatment per week for each patient (though it is currently unclear to what use this information will be put).
How do we start an ERD?
1. Define Entities: these are usually nouns used in descriptions of the system, in the discussion of business rules, or in documentation; identified in the narrative (see highlighted items above).
2. Define Relationships: these are usually verbs used in descriptions of the system
Data models are tools used in analysis to describe the data requirements and assumptions in the system from a top-down perspective. They also set the stage for the design of databases later on in the SDLC.
There are three basic elements in ER models:
Entities are the "things" about which we seek information.
Attributes are the data we collect about the entities.
Relationships provide the structure needed to draw information from multiple entities.
Generally, ERD's look like this: adapted from another professor.
Developing an ERD
Developing an ERD requires an understanding of the system and its components. Before discussing the procedure, let's look at a narrative created byProfessor Harman.
Consider a hospital:
Patients are treated in a single ward by the doctors assigned to them. Usually each patient will be assigned a single doctor, but in rare cases they will have two.
Heathcare assistants also attend to the patients, a number of these are associated with each ward.
Initially the system will be concerned solely with drug treatment. Each patient is required to take a variety of drugs a certain number of times per day and for varying lengths of time.
The system must record details concerning patient treatment and staff payment. Some staff are paid part time and doctors and care assistants work varying amounts of overtime at varying rates (subject to grade).
The system will also need to track what treatments are required for which patients and when and it should be capable of calculating the cost of treatment per week for each patient (though it is currently unclear to what use this information will be put).
How do we start an ERD?
1. Define Entities: these are usually nouns used in descriptions of the system, in the discussion of business rules, or in documentation; identified in the narrative (see highlighted items above).
2. Define Relationships: these are usually verbs used in descriptions of the system