Data modeling is a technique to document a software system using diagrams and symbols.
It is used to represent communication of data.
The highest level of abstraction for the data model is called the Entity Relationship Diagram (ERD).
It is a graphical representation of data requirements for a database.
Entity Relationship Diagram
The main value of carefully constructing an ERD is that it can readily be converted into a database structure.
There are three components in ERD.
- Entities: Number of tables you need for your database.
- Attributes: Information such as property, facts you need to describe each table.
- Relationships: How tables are linked together.
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,
locations, roles, events or things.
For example: students, courses, books, campus, employees, payment, projects.
A specific example of an entity is called an instance. Each instance becomes a
record or a row in a table.
For example: the student John Smith is a record in a table called students.
Relationships are the associations between the entities. Verbs often describe relationships between entities.
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 (1:1)
A single entity instance in one entity class is related to a single entity instance in another entity class.
- 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
- 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
Many to Many Relationship (M:M)
Each entity instance in one entity class is related to multiple entity instances in another entity class; and
- Each student can take many classes, and each class can be taken by many students.
- Each consumer can buy many products, and each product can be bought by many consumers.
The detailed Crow's Foot Relationship symbols can be found here. Crow's Foot
Many to many relationships are difficult to represent. We need to decompose a many to many (M:M) relationship
into two one-to-many (1:M) relationships.
Attributes are facts or description of entities. They are also often nouns and become the columns of the table.
For example, for entity student, the attributes can be first name, last name, email, address and phone numbers.
Primary Key* or identifier is an attribute or a set of attributes that uniquely identifies
an instance of the entity. For example, for a student entity, student
number is the primary key since no two students have the same student number.
We can have only one primary key in a table. It identify uniquely
every row and it cannot be null.
A foreign key+ (sometimes called a referencing key) is a key used to link two tables
together. Typically you take the primary key field from one table and insert it into the
other table where it becomes a foreign key (it remains a primary key in the original table).
We can have more than one foreign key in a table.
Here's a sample crowsfoot diagram from a past
offering of CS270 taught here at the University of Regina. We've redrawn the diagrams using more modern
diagramming tools, but the content is unchanged. It uses a lot of ERD symbols, so you might want to use Vivek Chawla's quick
guide while you read it.
- How many entities are there in this diagram and what are they?
- There are seven entities: STUDENT, COURSE, INSTRUCTOR, SEAT, CLASS, SECTION and PROFESSOR.
- What are the attributes for entity STUDENT?
- The attributes for Entity STUDENT are: student_id, student_name and student_address
- What is the primary key for STUDENT?
- The primary key for STUDENT is: student_id
- What is the primary key for COURSE?
- Not a trick question! There is only one primary key, but it is made up of two attributes. This is
called a compound key.
- What foreign keys do STUDENT and COURSE contain?
- STUDENT and COURSE contain no foreign keys in this diagram. This might suggest that there are
problems with the design... among them is the many to many relationship here. This usually requires that we
create a separate table to describe the relationship. This type of table usually connects foreign ids to each
In this case, let's add an entity called REGISTRATION in the middle of the "takes" relationship. Since
students probably sit in different seats for each course they are registered in, lets relate SEAT to
REGISTRATON instead of STUDENT:
The registration entity also uses a special case of compound key that includes compound foreign keys - it is a
composite key. [ref].
Many tools are available to help you design Entity Relationship Diagrams. Here is a short list of options you
might want to try.
- The REGISTRATION redesign diagram was done with draw.io.
draw.io is free on-line diagramming software that can be connected to cloud drives
like Google Drive, OneDrive, DropBox and GitHub.
- Once draw.io page is loaded, make sure to choose Entity Relation
on the left hand side.
- If you are lucky your lab instructor will do a demonstration... ;)
- To submit your completed ERD, export it to PDF like this: File->Export as->PDF...
Create an appropriate Entity Relationship Diagram (ERD) for the
data associated with an online application. This ERD should include the user accounts,
posts and re-posts entities. To get you started,
you can follow the example given below.
The User entity collect attributes: user_id(*primary key), user_email, user_name, user_password
user_DOB, User_image and etc.
The post entity collect attributes: post_id(*primary key), user_id (+foreign key), post_detail,
post_date, URL and etc.
What attributes should Re-post entity contain? Think of one or more things a repost can add to a post. Justify your design with a label.
The relationship shown between user and post in our example is good enough. Add relationships between Re-post and the other two. Justify or explain your decisions in text labels, and be sure you have selected compatible primary and foreign keys. For example:
A user may make many posts, but each post was written by only one user.
We may search for the user_id in the posts to find all the posts a user made.
Please export the file as a PDF document.
Upload the ERD on UR Courses and provide a working link to the ERD from your homepage before the deadline indicated in UR Courses for your section.
Be ready for next lab, get your MySQL account set up.
Marks will be deducted if your MySQL account is not ready by next lab.
Creating a MySQL Account
For storing records or data sets, the CS215 labs use WebDev to work with a MySQL database.
The following provides instructions to create your first MySQL database account.
- Go to the CS Account Services Web site and log in with your Campus Username and Password
- Click on the "MySQL" link
- If you don't have a database yet, please scroll down and read the notes about MySQL on WebDev.
- Select your MySQL password, then click Create Database
- Please make a note (on your phone or otherwise) of this password because you will be using it in a later lab.
- If an account already exists, you can reset your password or drop your database
- It takes 5 to 30 minutes to create a MySQL database.