This repository, SQL_Notes, is a well-organized collection of SQL problem solutions that primarily includes LeetCode SQL challenges alongside problems from other sources. It aims to be more than just a collection of answers; it also provides explanations and reminders of key SQL concepts, useful for both learning and refreshing your knowledge.

This repository will cover many important concepts, including:

• Basic SQL Syntax: SELECT, FROM, WHERE, ORDER BY, GROUP BY, HAVING
• Filtering Data: WHERE clause, comparison operators, logical operators (AND, OR, NOT), IN, BETWEEN, LIKE
• Joining Tables: INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN, self-joins
• Aggregate Functions: COUNT, SUM, AVG, MIN, MAX
• Grouping Data: GROUP BY clause, HAVING clause
• Subqueries: Using subqueries in SELECT, FROM, WHERE, and HAVING clauses
• Window Functions: OVER, PARTITION BY, ORDER BY within PARTITION, RANK, DENSE_RANK, ROW_NUMBER
• Common Table Expressions (CTEs): WITH clause
• Data Manipulation: INSERT, UPDATE, DELETE
• Data Definition: CREATE TABLE, ALTER TABLE, DROP TABLE, data types, constraints (PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL)
• Indexes: Creating and using indexes to improve query performance
• Transactions: ACID properties, BEGIN TRANSACTION, COMMIT, ROLLBACK
• String Functions: SUBSTRING, CONCAT, LENGTH, TRIM, UPPER, LOWER
• Date and Time Functions: Functions specific to your database system (e.g., MySQL's DATE(), YEAR(), MONTH(), DAY(), DATE_FORMAT(); PostgreSQL's date_part(), to_char())
• Conditional Logic: CASE statements
• NULL Handling: IS NULL, IS NOT NULL, COALESCE
• Set Operations: UNION, UNION ALL, INTERSECT, EXCEPT (or MINUS, depending on the database)