1️⃣ What is SQL?

SQL (Structured Query Language) is a standard language used to store, retrieve, update, and manage data in relational database systems such as MySQL, PostgreSQL, SQL Server, and Oracle.

2️⃣ Most Common SQL Commands

• SELECT – Fetch data from a table
• INSERT – Add new data
• UPDATE – Modify existing data
• DELETE – Remove data
• CREATE – Create tables or databases
• DROP – Delete tables or databases

3️⃣ Filtering Data

Use the WHERE clause to filter records based on specific conditions.

SELECT *
FROM employees
WHERE salary > 50000;

4️⃣ Sorting Data

Use ORDER BY to sort results in ascending (ASC) or descending (DESC) order.

SELECT name
FROM students
ORDER BY marks DESC;

5️⃣ Using Functions

Aggregate functions perform calculations on multiple rows.

• COUNT() – Counts number of records
• AVG() – Calculates average value
• MAX() / MIN() – Finds highest / lowest value
• SUM() – Calculates total sum

6️⃣ Grouping Data

GROUP BY is used with aggregate functions to group records.

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

7️⃣ Joins

Joins combine rows from multiple tables using related columns.

• INNER JOIN – Matching records in both tables
• LEFT JOIN – All records from left table + matches
• RIGHT JOIN – All records from right table + matches
• FULL JOIN – All records from both tables

8️⃣ Aliases

Aliases improve readability by renaming columns or tables temporarily.

SELECT name AS employee_name
FROM employees;

9️⃣ Subqueries

A subquery is a query nested inside another query.

SELECT name
FROM students
WHERE marks > (SELECT AVG(marks) FROM students);

🔟 Real-World Use Cases

• Managing employee records
• Generating sales and financial reports
• Tracking inventory levels
• Analyzing customer behavior and insights