COALESCE無法按預期工作（mysql）

Question

我正在嘗試寫每個月的銷售和利潤明細，並且合並功能無法按預期工作，即不會按我的意願更改NULL。

這是我的代碼：

SELECT  coalesce (extract(month from o.order_date),'Total year') AS mois, 
                sum(op.selling_price * op.quantity) AS 'Monthly sales',
                sum(op.selling_price * op.quantity) - sum(p.buying_price * op.quantity) AS 'Monthly Profit',
                count(r.return_id)
FROM order_products op
JOIN orders o
ON o.order_id = op.order_id
LEFT JOIN returns r
ON r.order_product_id = op.order_product_id 
JOIN products p
ON p.product_id = op.product_id
WHERE extract(year from o.order_date) = '2016'
GROUP BY mois
WITH ROLLUP;

運行此代碼時，最后一行（匯總）在“ mois”列下仍顯示“ NULL”。

我可能會錯過的任何想法嗎？

我已經嘗試過使用ifnull函數，但是遇到了同樣的問題。

謝謝！

Answer 1

使用匯總將查詢放在子查詢中，並在主查詢中使用COALESCE 。

SELECT COALESCE(mois, 'Total year') AS mois, `Monthly sales`, `Monthly Profit`, return_count
FROM (
    SELECT  extract(month from o.order_date) AS mois, 
                    sum(op.selling_price * op.quantity) AS 'Monthly sales',
                    sum(op.selling_price * op.quantity) - sum(p.buying_price * op.quantity) AS 'Monthly Profit',
                    count(r.return_id) AS return_count
    FROM order_products op
    JOIN orders o
    ON o.order_id = op.order_id
    LEFT JOIN returns r
    ON r.order_product_id = op.order_product_id 
    JOIN products p
    ON p.product_id = op.product_id
    WHERE extract(year from o.order_date) = '2016'
    GROUP BY mois
    WITH ROLLUP) AS x