MySQL查詢僅在行存在時返回值

Question

我有以下查詢，它可以完美運行，但前提是每個選擇都找到一行。

我嘗試添加IFNULL以返回0（如果未找到任何行），但仍無法獲得正確的返回值。

SELECT IFNULL(paid_value,0)-IFNULL(ordered_value,0)+IFNULL(credit_value,0) AS account_balance
            FROM
            (
              SELECT customer_id, SUM(order_total) AS ordered_value
              FROM orders
              WHERE customer_id = '1'
              GROUP BY customer_id
            ) AS orders
            LEFT JOIN
            (
              SELECT customer_id, SUM(amount) AS paid_value
              FROM transactions
              WHERE customer_id = '1'
              GROUP BY customer_id
            ) as payments
            ON orders.customer_id = payments.customer_id
            LEFT JOIN
            (
                SELECT customer_id, SUM(amount) AS credit_value
                FROM credits
                WHERE customer_id = '1'
                GROUP BY customer_id
            ) as credits
            ON orders.customer_id = credits.customer_id

該查詢當前返回空，不返回NULL或0。

當我跑步

              SELECT customer_id, SUM(order_total) AS ordered_value
              FROM orders
              WHERE customer_id = '1'
              GROUP BY customer_id

除非有一行，它還會返回空值，而不是NULL或0。 為了使完整查詢正常工作，三個獨立查詢中的每一個都需要在其中包含一行。

有任何想法嗎？

Answer 1

如果因為沒有一個列都沒有結果集，那么返回一個空結果集，如果您想始終在任何情況下都顯示一行，則可以嘗試使用以下技巧：

SELECT IFNULL(SUM(payments.paid_value),0)-IFNULL(SUM(orders .ordered_value),0)+IFNULL(SUM(credits.credit_value),0) AS account_balance
FROM 
(SELECT 1 AS idx, 0 AS paid_value, 0 AS ordered_value, 0 AS credit_value) a
LEFT JOIN
            (
              SELECT 1 AS idx, customer_id, SUM(order_total) AS ordered_value
              FROM orders
              WHERE customer_id = '1'
              GROUP BY customer_id
            ) AS orders
            ON a.idx = orders.idx 
            LEFT JOIN
            (
              SELECT customer_id, SUM(amount) AS paid_value
              FROM transactions
              WHERE customer_id = '1'
              GROUP BY customer_id
            ) as payments
            ON orders.customer_id = payments.customer_id
            LEFT JOIN
            (
                SELECT customer_id, SUM(amount) AS credit_value
                FROM credits
                WHERE customer_id = '1'
                GROUP BY customer_id
            ) as credits
            ON orders.customer_id = credits.customer_id
            GROUP BY a.idx

該示例是一種概念證明，甚至可以用於需要始終返回具有默認值的行（即使基礎表中沒有元素）的其他情況。

Answer 2

如果您想在沒有可用數據的情況下進行行事件，那么我想您應該擁有UNION 。 或者，在這種情況下，您可以包括客戶表，並將每個表放在右側，進行LEFT JOIN 。

我認為遵循選擇將為您提供一個理想的選擇。

SELECT O.customer_id, SUM(O.order_total) AS ordered_value
              FROM Customers C
              left join orders O on C.ID =o.customer_id
              WHERE O.customer_id = '1'
              GROUP BY O.customer_id