COALESCE 没有按预期工作（mysql）

Question

我正在尝试编写每月销售额和利润的明细，但合并功能没有按预期工作，即没有按照我的意愿更改 NULL。

这是我的代码：

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;

运行此代码时，最后一行（汇总）在“mois”列下仍显示“NULL”。

知道我可能错过了什么吗？

我尝试过使用 ifnull 函数，但遇到了同样的问题。

谢谢！

Answer 1

将使用汇总的查询放在子查询中，并在主查询中使用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