如何總是返回子查詢結果？

Question

我知道這個問題被問了很多，但無法弄清楚，如果我應該使用COALESCEifnull或別的東西。

我有以下查詢：

select (amount- 
(select (sum(l.amount_sum)-sum(lr.reward)) as total 
from logs l 
join logs_rewards lr on l.id = lr.related_log_id 
where l.agent_id = '1')) 
as balance 
from logs_payments 
where agent_id = '1' 

現在的問題是，如果agent_id不logs_payments存在的查詢將不返回行，但我想它做的是反正返回的結果，但如果amounts不存在，將其設置爲0的子查詢

現在我已經想過查詢不同的表和剛剛加盟的結果是這樣的：

select (lp.amount- 
(select (sum(l.amount_sum)-sum(lr.reward)) as total 
from logs l 
join logs_rewards lr on l.id = lr.related_log_id 
where l.agent_id = '1')) 
as balance 
from agents a 
join logs_payments lp on lp.agent_id = a.id 
where a.id = '1' 

但這也沒有奏效。我應該採取什麼方法解決這個問題？

爲了明確這一點，我想這個查詢總是返回子查詢的量，因此，如果不存在量仍然返回結果，但計算它爲0

Answer 1

嗯，我知道了使用JW的約LEFT JOIN評論工作和Saharsh的言論有關ifnull使用下面的代碼：

select ifnull(sum(lp.amount),0)- 
(select (sum(l.amount_sum)-sum(lr.reward)) as total 
from logs l 
join logs_rewards lr on l.id = lr.related_log_id 
where l.agent_id = '1') 
as balance 
from agents a 
left join logs_payments lp on lp.agent_id = a.id 
where a.id = '1' 

謝謝你們的幫助！

Answer 2

使用LEFT JOIN

SELECT a.amount - COALESCE(b.total, 0) as balance 
FROM logs_payments a 
     LEFT JOIN 
     (
      SELECT agent_id, sum(l.amount_sum) - sum(lr.reward) as total 
      FROM logs l 
        INNER JOIN logs_rewards lr 
         ON l.id = lr.related_log_id 
      GROUP BY agent_id 
     ) b ON a.agent_id = b.agent_id 
WHERE a.agent_id = 1 

Answer 3

試試這個：

SELECT l.agent_id, IFNULL(lp.amount, 0) amt, l.total 
FROM (SELECT l.agent_id, (SUM(l.amount_sum) - SUM(lr.reward)) AS total 
     FROM logs l INNER JOIN logs_rewards lr ON l.id = lr.related_log_id 
     WHERE l.agent_id = '1') AS l 
LEFT jon log_payments lp ON l.agent_id =lp.agent_id 
WHERE l.agent_id = '1'