当空切换到0时，mysql离开连接为空

Question

I have a query below and not all values in the sales_creditmemo table exist in the sales_order_item table, so a lot of the "Totaal_inclusief BTW en excl credit" is NULL. How can i take the c.base_grand_total as 0 instead of NULL, so the total is b.base_grand_total instead of NULL

SELECT a.order_id AS "Ordernummer", a.created_at AS "Orderdatum", 
b.base_grand_total AS "Inclusief BTW", b.base_tax_amount AS "Berekende BTW", 
c.base_grand_total AS  "Credit-terugbetaald",
(b.base_grand_total - c.base_grand_total) AS "Totaal_inclusief BTW en excl 
credit" FROM `sales_order_item` a 
INNER JOIN sales_invoice b ON a.order_id = b.order_id
LEFT JOIN sales_creditmemo c ON a.order_id = c.order_id
WHERE a.created_at > '2017-01-01'
GROUP BY a.order_id

Answer 1

By using the coalesce function:

SELECT  a.order_id AS "Ordernummer", 
        a.created_at AS "Orderdatum", 
        b.base_grand_total AS "Inclusief BTW", 
        b.base_tax_amount AS "Berekende BTW", 
        c.base_grand_total AS  "Credit-terugbetaald",
        (b.base_grand_total - coalesce(c.base_grand_total, 0)) AS "Totaal_inclusief BTW en excl 
credit" 
FROM `sales_order_item` a 
INNER JOIN sales_invoice b ON a.order_id = b.order_id
LEFT JOIN sales_creditmemo c ON a.order_id = c.order_id
WHERE a.created_at > '2017-01-01'
GROUP BY a.order_id