帶加減值的mysql連接查詢null

Question

如果makale_payments表爲空，則查詢返回null。如何設置查詢喲獲得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

即使專門測試了ISNULL，MySQL也不會返回返回空值的行的聚合值。 ISNULL僅用於檢查列是否爲空，而不是如果關係返回null。然而，我發現了一個（儘管很晦澀）的方法來解決這個限制。基本前提是聯合所有兩個sql語句並使用兩個聯合查詢作爲子查詢來從中進行選擇。

的聯合在一起queryies會的形式爲：

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 

有了上面的查詢，你會得到兩行，每行的關係不是零和一排當它。如果你將這些行相加並分組，那麼你應該得到你想要的值。所以，你再創建一個使用上面的表中，從選擇，歸納彙總列的查詢：

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 

我大概是掉在查詢的語法，對不起，我沒有的MySQL在這裏測試。

Answer 2

這是IFNULL是：

IFNULL(SUM(...), 0.0) 

Answer 3

一個簡單的WHERE不會傷害我猜

WHERE SUM(`article_payment`) > 0 

Answer 4

article_payment可能是NULL一些行。試試這個：

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