Introduction

SQL stands for Structured Query Language. It is the standard language used to communicate with databases.

With SQL, you can:

• Retrieve data from a database
• Add new data
• Update existing data
• Delete data
• Create and modify database structures

Think of SQL like a language for talking to data stored in tables.

1️⃣ What Is a Database?

A database is a collection of organized data.

Example: A company stores information about:

• Employees
• Customers
• Orders
• Products

2️⃣ SQL Basics: Tables

Data in a database is stored in tables.

A table is like a spreadsheet:

| EmployeeID | Name       | Department | Salary |
|------------|------------|------------|--------|
| 1          | John       | Sales      | 3000   |
| 2          | Mary       | HR         | 3500   |
| 3          | David      | IT         | 4000   |
| 4          | Anna       | Finance    | 3200   |

• Rows = records
• Columns = fields

3️⃣ Common SQL Commands

SQL commands are divided into categories:

| Category                           | Purpose                   | Example Command                       |
|-----------------------------------|---------------------------|---------------------------------------|
| DDL (Data Definition Language)     | Define database structure | CREATE TABLE, ALTER TABLE, DROP TABLE |
| DML (Data Manipulation Language)   | Add, update, delete data  | INSERT, UPDATE, DELETE                |
| DQL (Data Query Language)          | Retrieve data             | SELECT                                |
| DCL (Data Control Language)        | Control access            | GRANT, REVOKE                         |
| TCL (Transaction Control Language) | Manage transactions       | COMMIT, ROLLBACK                      |

4️⃣ Retrieving Data: SELECT Statement

The most basic SQL command is SELECT:

SELECT column1, column2
FROM table_name;

Example:

SELECT Name, Salary
FROM Employees;

Output:

| Name  | Salary |
|-------|--------|
| John  | 3000   |
| Mary  | 3500   |
| David | 4000   |
| Anna  | 3200   |

5️⃣ Filtering Data: WHERE Clause

To retrieve specific data, use WHERE:

SELECT Name, Salary
FROM Employees
WHERE Department = 'IT';

Output:

| Name  | Salary |
|-------|--------|
| David | 4000   |

6️⃣ Sorting Data: ORDER BY

Sort results ascending (ASC) or descending (DESC):

SELECT Name, Salary
FROM Employees
ORDER BY Salary DESC;

Output:

| Name  | Salary |
|-------|--------|
| David | 4000   |
| Mary  | 3500   |
| Anna  | 3200   |
| John  | 3000   |

7️⃣ Aggregating Data

SQL can calculate totals, averages, etc., using aggregate functions:

• COUNT() → count rows
• SUM() → total
• AVG() → average
• MAX() → maximum
• MIN() → minimum

Example:

SELECT AVG(Salary) AS AverageSalary
FROM Employees;

Output:

| AverageSalary |
|---------------|
| 3425          |

8️⃣ Grouping Data: GROUP BY

Use GROUP BY to summarize data by category.

Example: Average salary per department:

SELECT Department, AVG(Salary) AS AvgSalary
FROM Employees
GROUP BY Department;

Output:

| Department | AvgSalary |
|------------|-----------|
| Sales      | 3000      |
| HR         | 3500      |
| IT         | 4000      |
| Finance    | 3200      |

9️⃣ Joining Tables

Data is often in multiple tables. Use JOIN to combine them.

Example: Two tables

Employees Table

| EmployeeID | Name  | DepartmentID |
|------------|-------|--------------|
| 1          | John  | 1            |
| 2          | Mary  | 2            |
| 3          | David | 3            |
| 4          | Anna  | 4            |

Departments Table

| DepartmentID | DepartmentName |
|--------------|----------------|
| 1            | Sales          |
| 2            | HR             |
| 3            | IT             |
| 4            | Finance        |

SQL JOIN:

SELECT e.Name, d.DepartmentName
FROM Employees e
JOIN Departments d
ON e.DepartmentID = d.DepartmentID;

Output:

| Name  | DepartmentName |
|-------|----------------|
| John  | Sales          |
| Mary  | HR             |
| David | IT             |
| Anna  | Finance        |

🔟 Inserting Data

Add new records using INSERT:

INSERT INTO Employees (EmployeeID, Name, Department, Salary)
VALUES (5, 'Liam', 'Marketing', 3600);

1️⃣1️⃣ Updating Data

Change existing data using UPDATE:

UPDATE Employees
SET Salary = 3800
WHERE Name = 'Mary';

1️⃣2️⃣ Deleting Data

Remove records using DELETE:

DELETE FROM Employees
WHERE Name = 'Anna';