Data Modeling and Entity Relationship Diagram (ERD)

Data Modeling

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.

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, 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

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.

relationship

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.

For example:

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:

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.

For example:

The detailed Crow's Foot Relationship symbols can be found here. Crow's Foot Relationship Symbols

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

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

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.

Foreign key

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.

An Example

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.

crow's foot

Quick Questions

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 other.
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:
STUDENT-COURSE-SEAT relations re-designed as a REGISTRATION
The registration entity also uses a special case of compound key that includes compound foreign keys - it is a composite key. [ref].

ERD Tools

Many tools are available to help you design Entity Relationship Diagrams. Here is a short list of options you might want to try.

Visio

ConceptDraw

draw.io/diagrams.net

In Class Exercise Lab Assignment

Description

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.

User entity

The User entity collect attributes: user_id(*primary key), user_email, user_name, user_password user_DOB, User_image and etc.

Post entity

The post entity collect attributes: post_id(*primary key), user_id (+foreign key), post_detail, post_date, URL and etc.

Re-post entity

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.

Relationships

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 URcourses and provide a working link to the ERD from your homepage before 11:55pm on the day you took this lab.

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 Hercules to work with a MySQL database. The following provides instructions to create your first MySQL database account.

  1. Go to the secure CS Web site
  2. Click on the "MySQL" link

    secure CS Web site

  3. Enter your CS username/ password (the Titan password you use for this class), then click Login

    login

  4. If you don't have a database yet, please read the rules, accept the policy and click Process.
  5. Select your MySQL password (must be different from your CS password), then click Create
  6. Please make a note (on your phone or otherwise) of this password because you will be using it in a later lab.
  7. If an account already exists, you can reset your password or drop your database

    reset or drop DB

  8. It takes 5 to 30 minutes to create a MySQL database.