SQL - 計數與嵌套子查詢

Question

我有表的集合在關係數據庫

• products
• categories
• orders
• line_items
• customers

產品與類別具有多對多關係（連接表categories_products），並且也具有並屬於許多orders到line_items，這是products和orders的連接表，帶有一個id。 A customer也有很多orders。

我試圖把一些SQL，這將使我這種反應：

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 

這是SQL的巨厚片我一直在試圖用它來獲取這些值：

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 

只有計數本身是錯誤的。而不是獲取客戶在特定類別中購買的訂單項數量，而是對所有已完成訂單的LineItem進行計數。

如何才能讓計數正確地代表customer在category內購買的line_items的數量？

注意：在SQL文本中，o.status = 3正在使用枚舉來指示訂單是「完整的」。

Answer 1

我認爲你的內心加入categories_products是搞砸了。你應該設置一個小提琴，像@Strawberry建議的那樣，或者試試這個：

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

如果你想糾正你的計數，我會建議在子查詢中使用GROUP BY子句。如果您使用GROUP BY命令，那麼只有在查看用戶標識正確時才能獲取您檢索的特定訂單。我鼓勵你看看你的SQL代碼的其他部分的錯誤來清理這個龐大的查詢。例如，確保你想要使用不同的，並且你實際上想要使用左連接和內連接，這兩者都可能嚴重影響程序的性能。