SQL -- 使用嵌套子查询计数

Question

我在关系数据库中有一组表

• products
• categories
• orders
• line_items
• customers

Products 与类别具有多对多关系（连接表categories_products），并且还拥有并属于许多orders 到line_items，这是products 和orders 的连接表一个身份证。一个customer 也有很多orders。

我正在尝试整理一些 SQL 来给我这样的响应：

    customer_id | customer_first_name | category_id | category_name | number_purchased
    -----------------------------------
    1 |Jack | 1 | Electronics | 15
    2 |Jill | 1 | Electronics | 2
    2 |Jill | 2 | Hiking | 3

这是我一直试图用来获取这些值的大量 SQL：

    SELECT 
            DISTINCT customers.id AS customer_id,
            customers.first_name AS customer_first_name,
            categories.id AS category_id,
            categories.name AS category_name,
            (
                SELECT count(li.id) FROM line_items li
                INNER JOIN orders o ON li.order_id = o.id
                INNER JOIN products p ON li.product_id = p.id
                INNER JOIN categories_products cp ON cp.product_id = p.id
                WHERE
                    o.customer_id = customer_id 
                    AND o.status = 3
                    AND cp.category_id = category_id
            ) AS number_purchased
        FROM orders
        LEFT JOIN customers ON orders.customer_id = customers.id
        LEFT JOIN line_items li ON li.order_id = orders.id
        LEFT JOIN products ON products.id = li.product_id
        LEFT JOIN categories_products catpr ON catpr.product_id = products.id
        LEFT JOIN categories ON catpr.category_id = categories.id

只有计数本身是错误的。我不是获取客户在特定类别中购买的订单项的数量，而是获取已完成订单的所有订单项的计数。

如何让计数正确表示category 中特定customer 购买的line_items 数量？

注意：在 SQL 文本中，o.status = 3 使用枚举来指示订单“完成”。

Answer 1

我认为你与categories_products 的内部连接搞砸了。你应该像@Strawberry 建议的那样设置一个小提琴，或者试试这个：

SELECT 
        DISTINCT customers.id AS customer_id,
        customers.first_name AS customer_first_name,
        categories.id AS category_id,
        categories.name AS category_name,
        (
            SELECT count(li.id) FROM line_items li
            INNER JOIN orders o ON li.order_id = o.id
            INNER JOIN products p ON li.product_id = p.id
            WHERE
                o.customer_id = customer_id 
                AND o.status = 3
        ) AS number_purchased
    FROM orders
    LEFT JOIN customers ON orders.customer_id = customers.id
    LEFT JOIN line_items li ON li.order_id = orders.id
    LEFT JOIN products ON products.id = li.product_id
    LEFT JOIN categories_products catpr ON catpr.product_id = products.id
    LEFT JOIN categories ON catpr.category_id = categories.id

Answer 2

如果您想更正计数，我建议您在子查询中使用 GROUP BY 子句。如果您按订单分组，那么您只会在查看用户 ID 正确时获得您检索到的特定订单。我鼓励您查看 SQL 代码的其他部分中的错误，以清理这个庞大的查询。例如，确保您想要使用 distinct 并且您实际上想要使用左连接而不是内连接，这两者都可能严重影响程序的性能。