Entity Relationship Diagrams
Answer to Build an E-R diagram for Tiny College given the following information: Tiny College is divided into several schools: sch. Entity-Relationship Schema. Describes data requirements for a new information system. Direct, easy-to-understand graphical notation. Translates readily. One of the techniques that is great to use is the E-R diagram. In the above diagram regular entities are University, College, Dean, Professor.
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: Before discussing the procedure, let's look at a narrative created by Professor Harman. 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? Add attributes to the relations; these are determined by the queries,and may also suggest new entities, e.
College Entity Relationship Diagram Example
What questions can we ask? Which doctors work in which wards? There are three components in ERD. Number of tables you need for your database. Information such as property, facts you need to describe each table.
How tables are linked together. Entity Entities are the basic objects of ERDs. These are the tables of your database. Entity are nouns and the types usually fall into five classes.
Concepts, location, roles, events or things. Specific example of an entity is called an instance. Each isntance becomes a record or a row in a table. For example, the student John Smith is a record in a table called students. Relationships Relationships are the associations between the entities. Verbs often describe relationships between entities.
Data Modeling and Entity Relationship Diagram (ERD)
We will use Crow's Foot Symbols to represent the relationships. Three types of relationships are discussed in this lab. If you read or hear cardinality ratios, it also refers to types of relationships. One to One Relationship 1: Each student fills one seat and one seat is assigned to only one student. Each professor has one office space.
One to Many Relationship 1: M A single entity instance in one entity class parent is related to multiple entity instances in another entity class child For example: One instructor can teach many courses, but one course can only be taught by one instructor. One instructor may teach many students in one class, but all the students have one instructor for that class. Many to Many Relationship M: M Each entity instance in one entity class is related to multiple entity instances in another entity class; and vice versa.