Add advanced SQL queries

sql-queries/sql-queries-advanced/sql-queries-advanced.sql

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+-- Query with subquery and join
+SELECT *
+FROM employees e
+WHERE salary > (
+    SELECT AVG(salary)
+    FROM employees
+);
+
+-- Query with window functions
+WITH ranked_employees AS (
+    SELECT *,
+           ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank
+    FROM employees
+)
+SELECT *
+FROM ranked_employees
+WHERE rank <= 10;
+
+-- Query with CTE (Common Table Expression)
+WITH sales_summary AS (
+    SELECT department_id, SUM(amount) AS total_sales
+    FROM sales
+    GROUP BY department_id
+),
+average_sales AS (
+    SELECT AVG(total_sales) AS avg_sales
+    FROM sales_summary
+)
+SELECT s.department_id, s.total_sales, a.avg_sales
+FROM sales_summary s
+JOIN average_sales a ON s.department_id = a.department_id;