🧠 SQL Basics Cheatsheet 📊🛠️

1. What is SQL?
SQL (Structured Query Language) is used to store, retrieve, update, and delete data in relational databases.

2. Common SQL Commands:
- SELECT – Retrieves data
- INSERT INTO – Adds new data
- UPDATE – Modifies existing data
- DELETE – Removes data
- WHERE – Filters records
- ORDER BY – Sorts results
- GROUP BY – Aggregates data
- JOIN – Combines data from multiple tables

3. Data Types (Examples):
- INT, FLOAT, VARCHAR(n), DATE, BOOLEAN

4. Clauses to Know:
- WHERE – Filters rows
- LIKE, BETWEEN, IN, IS NULL – Conditional filters
- DISTINCT – Removes duplicates
- LIMIT – Restricts row count
- AS – Rename columns

5. SQL JOINS (Very Important):
- INNER JOIN – Matching rows in both tables
- LEFT JOIN – All from left + matches from right
- RIGHT JOIN – All from right + matches from left
- FULL OUTER JOIN – All rows from both tables

6. Aggregate Functions:
- COUNT(), SUM(), AVG(), MIN(), MAX()

7. Example Query:
SELECT name, AVG(score)
FROM students
WHERE grade = 'A'
GROUP BY name
ORDER BY AVG(score) DESC;

8. Constraints:
- PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, CHECK

9. Indexing & Optimization:
- Use INDEX to speed up queries
- Avoid SELECT * in production
- Use EXPLAIN to analyze query plans

10. Popular SQL Databases:
- MySQL, PostgreSQL, SQLite, Microsoft SQL Server, Oracle

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