Database Concepts - The Entity-Relationship Diagram

Via texample.net

In this post I will try to start database ideas with modeling. Here, I will explain what an ER diagram is and how it is used as a database model. In future posts I will explain more into detail of database ideas once I have established a basic bridge. Lets get started!


In order to continue understanding clearly upcoming concepts we need to first establish some more basic basic basic terms:

Primary Keys:
Primary Keys are the significant attributes of an entity. The primary keys are used to identify the specific entity, meaning that they are unique e.g., student IDs. You'll see more of an explanation of what an entity is below.

Foreign Keys:
Foreign Keys are specific attributes that are referencing an original attribute. Continuing with the student ID example, there may be a chart of students that are age 20, identified by their student IDs. That chart's student IDs refer to the original list of Students and their student IDs.
+-------------------+     +------------------------+
| Student |   ID#   |       |      aged 20 IDs       |
|   John    |  1001  |       |          1001              |
|   Sally   |  1002  |       +-----------------------+
+-------------------+

*see here that we have a roster of students, and then a list of students that are 20, we're assuming only John is 20. This is omitted but the roster of students should also include the details of their age.

Summarized:
Primary Keys are unique identifiers, Foreign keys refer to another entity's attributes. So, looking back into the previous post, we have a change.

1. Professors(Pid CHAR(50), Pname CHAR(50), Dept CHAR(50), PRIMARY KEY (Pid))
2. Students(Sid CHAR(50), Sname CHAR(50), Age INTEGER, MajorCHAR(50), PRIMARY KEY (Sid))
3. Departments(Did CHAR(50), Dname CHAR(50), PRIMARY KEY (Did))
4. Courses(Cno INTEGER, Cname CHAR(50), PRIMARY KEY (Cno))
5. EnrolledIn(Sid CHAR(50) REFERENCES Students(Sid), Cno INTEGER, PRIMARY KEY (Sid))

*here we have added things to specify PRIMARY KEY. Also, EnrolledIn has an addition to show that Sid is referring to the table Student(Sid).  With that said, we also need to establish some integrity constraints to avoid big disastrous problems. I'll talk about these once we get the whole picture.

The Entity-Relationship Diagram:
Also called ER diagrams. ER diagrams are used to plan out how different elements of the database are going to be connected and "mapped." They show the relationships between entities their attributes, as well as entity-entity relationships.

Entity:
The entities in the diagram above are the ones in blue(Employee, Salesman, Mechanic, etc.).
Entities are the "things" that are major in a database. In the diagram, we see that the "Important" objects that are the bigger part of this database are going to be the ones in blue. All of the entities are represented as a rectangle.

Attributes:
Attributes are the little details related to each entity.

And there you have it! If you read this then you have just learned the very basics of database concepts! I'm looking forward to writing more about this topic! See you soon.