带加减值的mysql连接查询null

Question

如果makale_payments表为空，则查询返回null。如何设置查询哟获得recors如果不存在SUM（payment_total）像0.00？

SELECT article_name,user_real_name,user_name,SUM(`article_payment`) as holding,paid 
FROM makale_articles AS t1 
JOIN makale_users ON (t1.article_name=makale_users.user_id) 
JOIN (SELECT user_id,SUM(`payment_total`) as paid FROM makale_payments GROUP BY user_id) AS t2 ON (t1.article_name=t2.user_id) 
GROUP BY t2.user_id 

Answer 1

即使专门测试了ISNULL，MySQL也不会返回返回空值的行的聚合值。 ISNULL仅用于检查列是否为空，而不是如果关系返回null。然而，我发现了一个（尽管很晦涩）的方法来解决这个限制。基本前提是联合所有两个sql语句并使用两个联合查询作为子查询来从中进行选择。

的联合在一起queryies会的形式为：

select column1, column2, SUM(column3) as payment from tablea, tableb where.... 
    union all 
    select column1, column2, 0.00 as payment from tablea --notice no join, and a specified value and alias for aggregate column 

有了上面的查询，你会得到两行，每行的关系不是零和一排当它。如果你将这些行相加并分组，那么你应该得到你想要的值。所以，你再创建一个使用上面的表中，从选择，归纳汇总列的查询：

select column1, column2, SUM(payment) from 

    (select column1, column2, SUM(column3) as payment from tablea, tableb where.... 
    union all 
    select column1, column2, 0.00 as payment from tablea) as b 
    group by column1, column2 

我大概是掉在查询的语法，对不起，我没有的MySQL在这里测试。

Answer 2

这是IFNULL是：

IFNULL(SUM(...), 0.0) 

Answer 3

一个简单的WHERE不会伤害我猜

WHERE SUM(`article_payment`) > 0 

Answer 4

article_payment可能是NULL一些行。试试这个：

SELECT 
    article_name, 
    user_real_name, 
    user_name, 
    SUM(COALESCE(`article_payment`, 0)) as holding, 
    paid 
FROM article_articles AS t1 
JOIN article_users ON (t1.article_name=article_users.user_id) 
JOIN (
    SELECT user_id, SUM(COALESCE(`payment_total`, 0)) as paid 
    FROM article_payments 
    GROUP BY user_id 
) AS t2 ON (t1.article_name=t2.user_id) 
GROUP BY t2.user_id