Introduction to SQL

SQL (Structured Query Language) is the standard language for interacting with relational databases such as MySQL, PostgreSQL, Oracle, SQL Server, and SQLite.

No matter if you are a data analyst, software developer, or database administrator, SQL is an essential skill.

1. SQL Basics (Beginner Level)

1.1 What is a Database?

A database is an organized collection of data. Relational databases store this data in tables, similar to Excel spreadsheets.

1.2 Basic SQL Syntax

SELECT column1, column2
  
FROM table_name
  
WHERE condition;

1.3 Common SQL Commands

• SELECT – Retrieves data
• INSERT – Adds new data
• UPDATE – Modifies existing data
• DELETE – Removes data
• CREATE – Creates database objects (tables, views)
• DROP – Deletes database objects

2. SQL Intermediate Concepts

2.1 Filtering & Sorting

SELECT * FROM employees
  
WHERE department = 'Sales'
  
ORDER BY salary DESC
  
LIMIT 10;

2.2 SQL Functions

• COUNT(), SUM(), AVG(), MIN(), MAX()

2.3 Grouping with GROUP BY

SELECT department, COUNT(*) AS total
  
FROM employees
  
GROUP BY department;

2.4 Table Joins

INNER JOIN Example:

SELECT e.name,e.name, d.department_name
  
FROM employees e
  
INNER JOIN departments d
  
ON e.dept_id = d.id;d.id;

3. SQL Table Operations

Creating a Table:

CREATE TABLE employees (
        
    id INT PRIMARY KEY,
        
    name VARCHAR(100),
        
    salary DECIMAL(10,2),
        
    dept_id INT
  
);

Inserting Data:

INSERT INTO employees (id, name, salary, dept_id)
  
VALUES (1, 'Alice', 55000, 2);

Updating & Deleting:

UPDATE employees SET salary = 60000 WHERE name = 'Alice';
  
DELETE FROM employees WHERE id = 1;

4. Advanced SQL Concepts

Subqueries:

SELECT name FROM employees
  
WHERE salary > (SELECT AVG(salary) FROM employees);

Window Functions:

SELECT name, salary,
              
       RANK() OVER (ORDER BY salary DESC) AS rank
  
FROM employees;

CTEs (Common Table Expressions):

WITH HighEarners AS (
    
    SELECT * FROM employees WHERE salary > 70000
  
)
  
SELECT * FROM HighEarners;

Stored Procedures (MySQL Example):

DELIMITER //

  
CREATE PROCEDURE GetHighEarners()
  
BEGIN
      
    SELECT * FROM employees WHERE salary > 70000;
  
END //

  
DELIMITER ;

Triggers:

CREATE TRIGGER before_insert
  
BEFORE INSERT ON employees
  
FOR EACH ROW
  
SET NEW.created_at = NOW();

5. SQL Performance Optimization

• Utilize appropriate indexes on columns used in WHERE, JOIN, or ORDER BY
-
• Avoid SELECT *,*; specify only the required columns
• Use EXPLAIN to analyze query performance

6. Real-World Use Cases

• Data Analysis – Aggregating sales, marketing, or user behavior data
• Web Development – Back-end CRUD operations
• ETL Pipelines – Extracting and transforming data for dashboards
• Reporting – Generating KPIs, metrics, and operational insights

SQL is one of the most sought-after skills in the data and software fields. Whether you’re analyzing data or developing scalable applications, mastering SQL will elevate your career.