使用 postgresql 进行 WITH CTE 查询

Question

我正在学习如何使用 SQL CTE，我想比较两个查询以获得相同的答案（使用 postgresql），但我失败了，有人可以帮忙吗？

我创建了这个查询，并获得了每部电影标题的总数（Sakila 数据库）：

SELECT COUNT(r.rental_id) rental_count, 
    f.title as "Film"
FROM film f 
JOIN inventory i 
ON f.film_id = i.film_id
JOIN rental r USING (inventory_id) 
GROUP BY f.title
ORDER BY rental_count DESC;

我想使用WITH（CTE）做同样的事情，为此我创建了以下代码：

WITH table1 AS (           
            SELECT f.film_id,
            f.title as "Film"
            FROM film f),

 table2 AS (           
        SELECT r.inventory_id,
        COUNT(r.rental_id) rental_count,
        i.film_id,
        i.inventory_id
        FROM inventory i
        JOIN rental r USING (inventory_id)
        GROUP BY r.inventory_id, i.film_id, i.inventory_id)

SELECT *
FROM table1
JOIN table2 
ON table1.film_id = table2.film_id;

问题在于，结果没有显示每部电影标题的总数，而是单独显示每部电影标题。

Answer 1

第二个 CTE 需要按薄膜分组才能产生等效的最终结果。

WITH table1 AS (
    SELECT
        f.film_id
      , f.title AS "Film"
    FROM film f
    )
, table2 AS (
    SELECT
        COUNT(r.rental_id) rental_count
      , i.film_id
    FROM inventory i
    JOIN rental r ON i.inventory_id = r.inventory
    GROUP BY i.film_id
    )
SELECT
      table2.rental_count
    , table1.Film
FROM table1
JOIN table2 ON table1.film_id = table2.film_id
ORDER BY rental_count DESC;

只是一个注释；我不建议在单个查询中同时使用自然和非自然连接类型，这可能会变得非常混乱。

SELECT
       COUNT(r.rental_id) rental_count
     , f.title AS "Film"
FROM film f
JOIN inventory i ON f.film_id = i.film_id
JOIN rental r ON i.inventory_id = r.inventory_id -- change here
GROUP BY f.title
ORDER BY rental_count DESC;