Gain insights into fundamental database concepts, relational databases, and entity-relationship diagrams. Learn about normalization techniques and basics of SQL to enhance database efficiency.

Databases are universal - they underlie the technology you use every day. They're a crucial component of everything from telecommunications systems, banking systems, and video games to just about any other software system that maintains some amount of persistent data.

In this course, you'll learn about the fundamental concepts of databases, why and when they're used, what relational databases are, and entity-relationship diagrams. You will also be exposed to techniques like normalization that help to increase the efficiency of databases. You'll wrap up with a look at the basics of Structured Query Language (SQL). 

After completing this course, you will be able to move onto more advanced courses involving database systems and SQL.

Database Design Fundamentals for Software Engineers

# Armstrong’s axioms

**Armstrong’s axioms** are a set of inference rules used to infer all the functional dependencies on a relational database. They were developed by William W. Armstrong in 1974.

We will denote a set of attributes by the letters X, Y, and Z. Also, we will represent the union of two sets of attributes X and Y by *XY* instead of the usual  X $\cup$ Y; this notation is rather standard in database theory when dealing with sets of attributes.

We will now highlight the three primary rules:

## Axiom of reflexivity

This axiom says, if Y is a subset of X, then X determines Y.

>  If Y $\subseteq$ X then , X $\rightarrow$ Y

For example, `Address` is composed of more than one piece of information; i.e. `House_No`, `Street`, and `State`. So according to the axiom of reflexivity `Address` (X) $\rightarrow$ `House_No` (Y) as `House_No` $\subseteq$ `Address`.

## Axiom of augmentation

The axiom of augmentation says if X determines Y, then XZ determines YZ for any Z.

> If X $\rightarrow$ Y, then XZ $\rightarrow$ YZ for any Z

The axiom of augmentation says that every non-key attribute must be fully dependent on the whole composite primary key. To get a better understanding, look at the example below:

| Std_Id    |   Course_Id   |  Name   |  Address   |  Age   |  Grade   |  Date_Completed   |
| --- | --- | --- | --- | --- | --- | --- |
|  1   |  CS-100   |  Bob   |  777 Brockton Avenue, Abington MA 2351 |  22   |  A-   |  2019-10-09   |
|  1   |  PHY-101   | Bob   |  777 Brockton Avenue, Abington MA 2351 |  22   | C-    |  2019-10-10    |
|  2   |  CS-100   |  Jack   |   30 Memorial Drive, Avon MA 2322 |   20  | B+    | 2019-10-12 |

In the table above, we can see that `Std_Id` and `Course_Id` form a composite key (as the combination of these two attributes can be used to identify each tuple uniquely). However, we can also observe that the `Name`, `Address`, and `Age` attributes are only dependent on the `Std_Id`, not on the whole `Std_Id` and `Course_Id` composite key.

This situation is not desirable because every non-key attribute has to be fully dependent on the PK not just part of it. In this situation, student information is only partially dependent on the PK (`Std_Id`) which violates the axiom of augmentation.

We will discuss the solution to this problem when we study [normalization](https://www.educative.io/courses/database-design-fundamentals/what-is-normalization) in the next chapter.


## Axiom of transitivity

The axiom of transitivity, also known as a **transitive dependency**, says if X determines Y, and Y determines Z, then X must also determine Z.

> If X $\rightarrow$ Y and Y $\rightarrow$ Z, then X $\rightarrow$ Z.

Let's consider the following example:

| Std_Id    |  Std_Name   |  Address   |  Age   | Prog_Id  | Prog_Name |
| --- | --- | --- | --- | --- | --- |
|  1   |  Bob   |  777 Brockton Avenue, Abington MA 2351 |  22   |  CS-200   | Introduction to Programming   |
|  1   |  Bob   |  777 Brockton Avenue, Abington MA 2351 |  22   |  PHY-100    | Modern Physics    |
|  2   |  Jack   |   30 Memorial Drive, Avon MA 2322 |   20  |  CS-300    | Advanced Programming |

The table above has information not directly related to the student; for instance, `Prog_Name` is not dependent on `Std_Id`; it’s dependent on `Prog_Id`.

This situation is not desirable because a non-key attribute (`Prog_Name`) depends on another non-key attribute (`Prog_Id`), which results in transitive dependency.

Again we will discuss the solution to this problem when we study [normalization](https://www.educative.io/courses/database-design-fundamentals/what-is-normalization) in the next chapter.



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In the next lesson, we will look at how to display these dependencies in diagrammatic form.



In this lesson, we will take a look at the different rules of FDs.

Introduction

Fundamental Concepts

Data Modeling

Entity-Relationship Data Model

The Relational Data Model

Functional Dependencies

Normalization

Structured Query Language

Conclusion

Rules of Functional Dependencies

Armstrong’s axioms

Axiom of reflexivity