SQL Basics

Before writing powerful SQL queries, it is very important to understand how data is stored inside a database. SQL works on a structured model, and once you understand this structure, everything else becomes easy.

Understanding Tables

A table is the most important object in a relational database. All data in SQL databases is stored inside tables.

You can think of a table like a spreadsheet:

• Each table has a name
• Each table contains rows and columns
• Each table stores data about one specific subject

Rows and Columns Explained

Let’s break down the structure of a table:

Visualizing a Table

Consider the following employees table:

+----+----------+------------+--------+
| id | name     | department | salary |
+----+----------+------------+--------+
| 1  | Alice    | HR         | 60000  |
| 2  | Bob      | IT         | 75000  |
| 3  | Charlie  | Finance    | 70000  |
+----+----------+------------+--------+
  

In this table:

• Each row represents one employee
• Each column represents a specific attribute
• The id column uniquely identifies each row

What is a Primary Key?

A primary key is a column (or group of columns) that uniquely identifies each row in a table.

Key characteristics of a primary key:

• It must be unique
• It cannot be NULL
• Each table should have one primary key

In the employees table, the id column is a primary key.

Basic SQL Statement Structure

Most SQL queries follow a simple and readable structure:

SELECT column_name
FROM table_name;
  

This structure clearly answers two questions:

• What data do you want? → SELECT
• From where? → FROM

Example: Selecting Data from a Table

To view all columns and rows from the employees table:

SELECT * FROM employees;
  

Explanation:

• SELECT * means select all columns
• FROM employees specifies the table name

Why This Structure Matters

Understanding tables, rows, and columns helps you:

• Write correct SQL queries
• Design clean databases
• Avoid data duplication
• Understand joins and relationships later

Every advanced SQL concept builds on these basics, so take time to understand them well.

What’s Next?

In the next lesson, we will focus completely on the SELECT statement and learn how to retrieve specific columns and format query results.