윈도우 함수

-- ROW_NUMBER
SELECT name, salary,
    ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank
FROM employees;

-- RANK and DENSE_RANK
SELECT name, dept, salary,
    RANK() OVER (PARTITION BY dept ORDER BY salary DESC) AS dept_rank,
    DENSE_RANK() OVER (PARTITION BY dept ORDER BY salary DESC) AS dense_rank
FROM employees;

-- LAG and LEAD
SELECT date, revenue,
    LAG(revenue, 1) OVER (ORDER BY date) AS prev_day,
    LEAD(revenue, 1) OVER (ORDER BY date) AS next_day,
    revenue - LAG(revenue, 1) OVER (ORDER BY date) AS diff
FROM daily_sales;

-- Moving average
SELECT date, revenue,
    AVG(revenue) OVER (
        ORDER BY date
        ROWS BETWEEN 6 PRECEDING AND CURRENT ROW
    ) AS moving_avg_7d
FROM daily_sales;