What is a database?

Data

Data is a raw fact.

For example:

On their own, these are only data.

But if we say:

The data now has meaning.

Information

Information is data that has meaning in a specific context and can be used.

Simply:

Data = a raw fact
Information = data that has gained meaning

The more common computer-science term for Database is “database” (پایگاه داده in Persian).

Some Persian sources also say “بانک اطلاعاتی”, but this lesson uses پایگاه داده.

Simple definition

A database is an organized collection of related data, kept so we can store, manage, and retrieve information.

A university might keep:

• students
• teachers
• courses
• enrollments
• grades

These facts are related. For example:

That related set of data can live in one database.

In SQL we usually work with a relational database.

Related data is typically stored in a table.

For example, a students table:

Three important terms:

Table

The whole structure above is a table. For example:

Row

Each row usually represents one entity or record. For example:

That row is one student.

Another common word: record.

Column

Each column holds one attribute. For example:

So:

No. This is an important mistake.

A database is not a table.

One database may contain hundreds of tables.

For example, a university database:

So:

A table is part of a database.

For example:

Imagine the university has no database. The education office has a file:

Inside it:

The exams office has its own file:

And student details are written again:

Finance has another file:

And again:

Sara’s information now exists in three places.

This approach is usually called a file-based system.

Some notes may say:

They usually mean:

Non-banking system

That is a file-based system.

Data lives in separate files. For example:

Banking system

The more precise term is a database system.

Data is managed in one database.

Applications do not each manage their own separate file.

Suppose Sara’s city is stored in three files:

Sara moves from Tehran to Isfahan. Education updates its file:

But exams forgets:

Now the system has two answers about one student:

This is one of the most important file-system problems.

English term: redundancy.

It means:

The same fact is stored more than once, without needing to be.

For example:

stored in three different files. That is redundancy.

The main problem is not only extra space. The copies can drift apart.

Term: data inconsistency.

For example:

Both talk about the same person, but the facts differ.

So we have an important relationship:

That is: redundancy can cause data inconsistency.

No. Learn this sentence carefully:

A database system usually controls and reduces unnecessary redundancy. It does not delete every repeated value in every case.

Sometimes data is repeated on purpose for technical reasons.

So do not say a database has no duplicate data. Say it helps control unnecessary redundancy.

Term: data integrity.

Data in the system must be valid, follow the rules, and keep logical relationships.

The university says:

So:

is valid. But:

is not valid.

Integrity examples

Example 1

That is illogical.

Example 2

A student tries to enroll in a course that does not exist:

But course 900 is not in the course table. That value must be rejected.

Example 3

A national ID must be unique. We do not want:

A DBMS can prevent this.

We will learn these more precisely in SQL. For now, just know the names.

Domain integrity

A column value must stay in a valid range. For example:

Entity integrity

Every record must be identifiable. Each student has a unique id.

Referential integrity

Links between tables must be valid. If we say:

a student with id=25 must actually exist.

Term: confidentiality.

It means:

Information is available only to people who are allowed to see it.

At a university a student may see:

but must not see:

An education clerk may see:

but should not necessarily see:

Do not mix these two.

Integrity

The question is:

Is the data correct and valid?

For example:

has an integrity problem.

Confidentiality

The question is:

Who is allowed to see the data?

A student who sees another student’s grades has a confidentiality problem.

So:

Term: Database Management System (DBMS).

A DBMS is software that manages the database.

A simple picture:

The application usually talks to the data through the DBMS.

This is one of the most important points.

Database

The data itself.

DBMS

The software that manages that data.

An analogy:

The books hold the information. The librarian decides where a book is, who may take it, who may access it, and how it is recorded.

The main jobs:

1. Storage

Store data

2. Retrieval

Fetch data. For example:

3. Insert

Add data

4. Update

Change data

5. Delete

Remove data

6. Integrity

Enforce validity rules

7. Security / access control

Control who can do what

8. Concurrency control

Handle many users at once

9. Backup & recovery

Back up and restore data

Term: concurrency.

Many users work with the system at almost the same time.

Imagine course registration. A thousand students are in the system at once.

The DBMS must keep these operations from corrupting the data.

You will see these names often:

These are database management systems.

More precisely, most of them are relational database management systems (RDBMS).

RDBMS means Relational Database Management System.

It manages relational data, usually as tables and the links between them. For example:

and they are related.

SQL means Structured Query Language.

SQL is neither a database nor a DBMS. It is a language.

We use SQL to tell the DBMS what to do. For example:

In plain words:

Show me all students.

Keep this picture in mind:

You write:

The DBMS receives the command, looks in the database, and returns the result.

Suppose our database has this table:

You tell the DBMS:

The DBMS checks the data and returns:

So:

Example:

Now we can say more clearly why databases were created.

A file system can have problems like:

• Data redundancy
• Data inconsistency
• Difficult data sharing
• Weak integrity control
• Weak access control
• Concurrency problems
• Difficult backup and recovery

A DBMS was built to manage these issues better.

Learn this part well:

And from the other side:

Suppose we have:

If Sara’s data is stored in five separate files:

First problem: redundancy — the data is repeated.

If one file changes and the others do not: inconsistency.

If someone enters:

Problem: integrity.

If a student can see everyone’s grades: confidentiality / access control.

We use a DBMS to manage this better, and SQL to talk to a relational DBMS.

Learn these words in both Persian and English:

Question 1

A student’s data is stored in three different files. What is the problem?

A) Integrity
B) Redundancy
C) Confidentiality

Answer: B — Redundancy

Question 2

One file says the city is Tehran and another says Isfahan. What is the problem?

Data inconsistency

Question 3

A student has grade 125 but grades must be between 0 and 20. What is the problem?

Integrity

Question 4

Ali can see Sara’s grade without permission. What is the problem?

Confidentiality / access control

Question 5

What is PostgreSQL?

A) Database
B) DBMS
C) SQL

Answer: B

Question 6

What is SQL?

A language for working with relational database management systems.

If you fully understand these four lines, you have the lesson:

Database is where data is kept in an organized way.
DBMS is the software that manages the database.
SQL is the language we use to ask the DBMS to do work.
RDBMS is a kind of DBMS that manages relational data and tables.

For example:

Almost every later SQL lesson is built on this foundation.