Introduction to SQL

SQL, which stands for Structured Query Language, is the standard language used to communicate with relational databases. Whether you are building a web application, analyzing business data, or managing records in a large enterprise system, SQL is the tool that allows you to store, retrieve, update, and delete data efficiently.

If you have ever wondered how websites display your profile information, how online stores keep track of inventory, or how banks process millions of transactions every day, the answer almost always involves SQL working behind the scenes.

This guide serves as the starting point of your SQL learning journey. By the end of this page, you will understand what SQL is, why it matters, and what you will learn throughout this course.

What Is SQL?

SQL is a declarative programming language designed specifically for managing data held in a relational database management system (RDBMS). Unlike general-purpose programming languages such as Python or JavaScript, SQL focuses on one thing: working with data.

When you write SQL, you describe what you want to get from the database, not how to get it. The database engine figures out the most efficient way to execute your request.

Here is a simple example of an SQL statement:

SELECT first_name, last_name
FROM employees
WHERE department = 'Engineering';

This query asks the database to return the first and last names of all employees who work in the Engineering department. You did not need to write loops, open files, or manage memory. You simply described the result you wanted, and the database delivered it.

Why Learn SQL?

SQL has been around since the 1970s, and it remains one of the most in-demand skills in the technology industry. Here is why learning SQL is a valuable investment:

• Universal adoption: Nearly every modern application uses a relational database. MySQL, PostgreSQL, SQL Server, Oracle, and SQLite are just a few examples of database systems that all use SQL.
• Career relevance: Roles such as backend developer, data analyst, data engineer, database administrator, and even product manager frequently require SQL knowledge.
• Simplicity and power: SQL has a relatively small set of commands, but they can express extremely complex data operations. You can go from writing your first query to producing advanced reports in a short time.
• Longevity: Technologies come and go, but SQL has remained the standard for decades. Learning SQL is an investment that will pay off throughout your entire career.

tip

SQL is consistently ranked among the top 3 most requested languages in job postings for technical roles. Even if you are not aiming to become a database specialist, knowing SQL will give you a significant advantage.

How SQL Interacts with Databases

To understand SQL properly, you need to know where it fits in the bigger picture. Here is a simplified view of how things work:

1. You write an SQL query using a database client, application code, or command-line tool.
2. The query is sent to the database server, which is the software (like PostgreSQL or MySQL) that manages your data.
3. The database engine parses and optimizes your query, figuring out the best way to execute it.
4. The engine executes the query against the stored data.
5. The results are returned to you in the form of a structured result set (rows and columns).

You do not interact with the files on disk directly. SQL acts as the intermediary language between you and the raw data.

What Can You Do with SQL?

SQL is often divided into several categories based on the type of operation being performed. Understanding these categories early will help you see the full scope of what SQL can accomplish.

Data Querying (DQL)

The most common use of SQL is reading data. The SELECT statement is the cornerstone of SQL and allows you to retrieve exactly the data you need.

SELECT product_name, price
FROM products
WHERE price > 50
ORDER BY price DESC;

This retrieves all products priced above 50, sorted from most expensive to least expensive.

Data Manipulation (DML)

Beyond reading, SQL lets you add, update, and remove data in your tables.

-- Insert a new record
INSERT INTO products (product_name, price) VALUES ('Wireless Mouse', 29.99);

-- Update an existing record
UPDATE products SET price = 24.99 WHERE product_name = 'Wireless Mouse';

-- Delete a record
DELETE FROM products WHERE product_name = 'Wireless Mouse';

Data Definition (DDL)

SQL is also used to create and modify the structure of your database, including tables, columns, and constraints.

CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(255) UNIQUE
);

Data Control (DCL)

SQL provides commands to manage access and permissions, controlling who can read or modify data.

GRANT SELECT ON customers TO analyst_role;
REVOKE DELETE ON customers FROM intern_role;

Transaction Control (TCL)

When you need to ensure that a group of operations either all succeed or all fail, SQL provides transaction control statements.

BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE account_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE account_id = 2;
COMMIT;

info

You do not need to memorize these categories right now. As you progress through the course, each section will cover them in detail with hands-on examples.

SQL Is Declarative, Not Procedural

One of the most important concepts to grasp early is that SQL is a declarative language. This means you tell the database what result you want, and the database decides how to get it.

Compare this with a procedural approach in a language like Python:

Procedural approach (Python-like pseudocode):

results = []
for row in all_employees:
    if row.department == 'Engineering':
        results.append(row.first_name)
results.sort()

Declarative approach (SQL):

SELECT first_name
FROM employees
WHERE department = 'Engineering'
ORDER BY first_name;

Both achieve the same result, but the SQL version does not specify how to loop through the data or how to sort it. The database engine handles those details internally, often in a highly optimized way.

This declarative nature is what makes SQL both accessible for beginners and powerful for experts. You can write a simple query on day one, yet the same language scales to handle queries over billions of rows in production databases.

What You Will Learn in This Course

This course is structured to take you from zero SQL knowledge to a confident, capable SQL user. Here is an overview of what lies ahead:

Foundations

You will start by understanding how databases work, setting up your own SQL environment, and learning about the differences between popular SQL dialects like MySQL, PostgreSQL, and SQL Server.

Querying Data

The core of SQL begins with SELECT statements. You will learn how to filter rows with WHERE, sort results with ORDER BY, eliminate duplicates with DISTINCT, use pattern matching with LIKE, and paginate results with LIMIT and OFFSET.

Aggregations and Grouping

You will discover how to summarize data using aggregate functions like COUNT, SUM, AVG, MIN, and MAX, and how to group results with GROUP BY and filter groups with HAVING.

Joining Tables

Real-world databases rarely consist of a single table. You will learn how to combine data from multiple tables using INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN, and more.

Subqueries and Set Operations

You will learn to nest queries inside other queries and use set operations like UNION, INTERSECT, and EXCEPT to combine result sets.

Modifying Data

Beyond reading data, you will learn to insert, update, delete, and upsert records safely and efficiently.

Database Structures

You will learn how to create databases and tables, define constraints, alter existing structures, and understand the principles of normalization and ER diagrams.

Built-in Functions

SQL comes with a rich set of built-in functions for working with strings, numbers, dates, and handling null values. You will also learn conditional logic with CASE WHEN.

Window Functions

One of the most powerful features in modern SQL, window functions allow you to perform calculations across sets of rows related to the current row without collapsing the result set.

CTEs and Views

Common Table Expressions (CTEs) and views let you write cleaner, more maintainable queries. You will also explore recursive CTEs and materialized views.

Indexes and Performance

Writing correct SQL is important, but writing fast SQL matters too. You will learn about indexes, execution plans, and query optimization best practices.

Transactions

You will understand the ACID properties that ensure data integrity and learn how to use transactions, isolation levels, and locking mechanisms.

Stored Procedures, Functions, and Triggers

You will learn to encapsulate reusable logic inside the database using stored procedures, user-defined functions, and triggers.

Security and Access Control

Finally, you will learn how to manage users, roles, and permissions, and how to protect your applications from SQL injection attacks.

note

Each section builds on the previous one. While you can jump to specific topics if you already have some experience, beginners are encouraged to follow the course in order for the best learning experience.

Prerequisites

This course is designed for entry-level to mid-level developers. You do not need any prior database experience. However, the following will help:

• Basic computer literacy: You should be comfortable using a terminal or command-line interface.
• Logical thinking: SQL relies on logic (AND, OR, NOT, comparisons). If you are comfortable with basic logical operations, you are ready.
• A willingness to practice: SQL is best learned by writing queries. Each lesson includes examples you can and should run on your own machine.

You do not need to know any programming language before starting this course. SQL stands on its own as a complete language for data management.

Your First Look at SQL

To close this introduction, here is a slightly more involved example that previews several concepts you will learn throughout this course. Do not worry about understanding every part of it right now. The goal is simply to show you what SQL looks like in practice.

SELECT
    d.department_name,
    COUNT(e.employee_id) AS total_employees,
    ROUND(AVG(e.salary), 2) AS average_salary
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
WHERE e.hire_date >= '2020-01-01'
GROUP BY d.department_name
HAVING COUNT(e.employee_id) > 5
ORDER BY average_salary DESC;

What this query does:

Clause	Purpose
SELECT	Chooses the columns and calculations to display
FROM / JOIN	Combines data from the employees and departments tables
WHERE	Filters to only employees hired since 2020
GROUP BY	Groups the results by department
HAVING	Keeps only departments with more than 5 employees
ORDER BY	Sorts by average salary, highest first

By the time you finish this course, queries like this will feel natural and intuitive.

Let's Get Started

You now have a clear picture of what SQL is, why it matters, and what this course will cover. The next step is to dive into the foundations: understanding how databases work, setting up your environment, and writing your very first query.

Head over to the next lesson to begin your journey into the world of relational databases and SQL.

tip

The best way to learn SQL is by doing. As you move through each lesson, open your database client and type out the examples yourself. Reading alone will not build the muscle memory you need to become proficient.