SQL stands for Structured Query Language. It's a standardized programming language designed for managing and manipulating relational databases. SQL allows you to create, read, update, and delete data in databases.

SQL commands are categorized into: DDL (Data Definition Language) - CREATE, ALTER, DROP; DML (Data Manipulation Language) - SELECT, INSERT, UPDATE, DELETE; DCL (Data Control Language) - GRANT, REVOKE; TCL (Transaction Control Language) - COMMIT, ROLLBACK, SAVEPOINT.

WHERE clause filters rows before grouping, while HAVING clause filters groups after GROUP BY. WHERE cannot use aggregate functions, but HAVING can. Example: WHERE salary > 50000 vs HAVING COUNT(*) > 5.

INNER JOIN returns matching records from both tables. LEFT JOIN returns all records from left table and matching from right. RIGHT JOIN returns all from right and matching from left. FULL OUTER JOIN returns all records when there's a match in either table.

A self-join is when a table is joined with itself. It's useful for hierarchical data like employee-manager relationships. Example: SELECT e1.name, e2.name as manager FROM employees e1 LEFT JOIN employees e2 ON e1.manager_id = e2.employee_id

Window functions perform calculations across a set of rows related to the current row without collapsing the result set. Unlike aggregate functions with GROUP BY, window functions preserve individual rows. Examples: ROW_NUMBER(), RANK(), SUM() OVER().

CTEs are temporary named result sets that exist within the scope of a single SQL statement. They improve readability and can be recursive. Syntax: WITH cte_name AS (SELECT ...) SELECT * FROM cte_name. Useful for complex queries and hierarchical data.