SQL window function 並加入

Question

我的查詢如下：

SELECT 
    category_name, film_title, rental_count
FROM
    (SELECT 
        c.name AS category_name, f.title AS film_title, 
        COUNT(r.rental_date) OVER (PARTITION BY f.title) AS rental_count
    FROM 
        category c
    JOIN 
        film_category fc ON fc.category_id = c.category_id
    JOIN 
        film f ON fc.film_id = f.film_id
    JOIN 
        inventory i ON i.film_id = f.film_id
    JOIN 
        rental r ON i.inventory_id = r.inventory_id) t1
WHERE 
    category_name IN ('Animation', 'Children', 'Classics', 'Comedy', 'Family', 'Music')
ORDER BY 
    1, 2

結果如下：

如您所見，電影的標題重復了，正如您已經知道的那樣，它不應該重復。 但是，我不明白為什么會這樣？

答案其實應該如上圖。

非常感謝任何幫助。

Answer 1

為什么會這樣？ 這就是 window 函數的作用。 它們不會減少行數。 在現有行上添加新列（通常是從多行收集的數據）。

您只需要一個聚合：

SELECT c.name AS category_name, f.title AS film_title, 
       COUNT(r.rental_date) as rental_count
FROM category c JOIN
     film_category fc
     ON fc.category_id = c.category_id JOIN
     film f
     ON fc.film_id = f.film_id JOIN
     inventory i
     ON i.film_id = f.film_id JOIN
     rental r
     ON i.inventory_id = r.inventory_id
WHERE c.category_name IN ('Animation', 'Children', 'Classics', 'Comedy', 'Family', 'Music')
GROUP BY 1, 2