====== Introduction to SQL ======


==== What is SQL? ====


Structured Query Language (SQL) is a standard programming language used to manage and manipulate relational databases. SQL is essential for tasks such as querying data, updating records, managing access control, and defining database structures.

SQL operates on structured data stored in tables, consisting of rows and columns. It provides various commands to interact with the database, ensuring efficient data retrieval and management.

==== History and Evolution ====


SQL was first developed in the 1970s at IBM for their System R project, based on Edgar F. Codd's relational model. Over time, SQL has evolved into an ANSI and ISO standard, with popular implementations like MySQL, PostgreSQL, Microsoft SQL Server, and Oracle Database.

Key milestones in SQL evolution:

**SQL-86:** The first standardized version.

**SQL-92:** Added better support for constraints and joins.

**SQL:1999:** Introduced features like triggers, recursive queries, and procedural elements.

**SQL:2003:** Included XML-related features.

**SQL:2011** and later: Added temporal data support and performance improvements.

==== Types of SQL Commands ====


SQL commands are categorized into five main types:

DDL (Data Definition Language) – Defines database structures

''CREATE TABLE'' – Creates a new table

''ALTER TABLE'' – Modifies an existing table

''DROP TABLE'' – Deletes a table

**Example:**

<code>
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department VARCHAR(50),
    salary DECIMAL(10,2)
);
</code>

**Example: Modifying a Table**

<code>
ALTER TABLE employees ADD COLUMN hire_date DATE;
</code>

**DML (Data Manipulation Language)** – Manipulates data

''INSERT'' – Adds records

''UPDATE'' – Modifies records

''DELETE'' – Removes records

**Example:**

<code>
INSERT INTO employees (id, name, department, salary) VALUES (1, 'John Doe', 'IT', 60000);
</code>

**Example: Updating a Record**
<code>
UPDATE employees SET salary = 70000 WHERE id = 1;
</code>

**DCL (Data Control Language)** – Manages access control

''GRANT'' – Assigns privileges

''REVOKE'' – Removes privileges

**Example:**

<code>
GRANT SELECT, INSERT ON employees TO 'user1'@'localhost';
</code>

TCL (Transaction Control Language) – Manages transactions

''COMMIT'' – Saves changes

''ROLLBACK'' – Undoes changes

**Example:**
<code>
BEGIN TRANSACTION;
UPDATE employees SET salary = 70000 WHERE id = 1;
ROLLBACK;
</code>

**DQL (Data Query Language)** – Retrieves data

''SELECT'' – Queries data

**Example:**
<code>
SELECT * FROM employees WHERE department = 'IT';
</code>