在mysql中如何从子查询返回零而不是null

我创建了一个查询，当从子查询返回null时，我希望返回0，我使用了IFNULL函数，但最后仍然得到null。下面是一个类似的查询，请帮忙，不知道该怎么办。（注：我用电话写了这篇文章，所以mu格式化可能看起来不好，对不起）

试试这个：

SELECT t1.id, t1.Amount, t1.parent, 
   IF(SUM(t2.amount) IS NULL, 0 ,SUM(t2.amount)) AS amount FROM transactions t2 WHERE t1.`id` = t2.parent GROUP BY t2.parent ) AS AmountRepaid, 
  ( SELECT CASE WHEN CAST(SUM(t3.amount) AS DECIMAL) > CAST(t1.amount AS DECIMAL) THEN "Over" WHEN CAST(SUM(t3.amount) AS DECIMAL) = CAST(t1.amount AS DECIMAL) THEN "Complete" ELSE "Incomplete" END FROM transactions t3 WHERE t1.`id` = t3.Parent GROUP BY t3.Parent ) AS PaymentStatus 
  FROM transactions t1 WHERE t1.`Parent` IS NULL;

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尝试使用以下方法：

SELECT t1.id, t1.Amount, t1.parent, 
       @Total := IFNULL( ( SELECT SUM(t2.amount) AS amount FROM transactions t2 WHERE t1.`id` = t2.parent GROUP BY t2.parent ), 0) AS AmountRepaid, 

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@jamesokpegorge：在子查询中使用

groupby

，因此在本例中，该子查询可以返回0行（如果没有

t1.id=t2.parent

），因此在本例中

@total

为空<代码>分组依据此处无效，因为您总是只选择一个组。因此，您可以删除GROUP BY以使用您的查询或我的答案中的use way。

SELECT t1.id, t1.Amount, t1.parent, 
       @Total := IFNULL( ( SELECT SUM(t2.amount) AS amount FROM transactions t2 WHERE t1.`id` = t2.parent GROUP BY t2.parent ), 0) AS AmountRepaid,