mysql - 列名前的 '0' 的用途

Question

我只是想知道，列名称前的 0 的目的是什么，因为我是这个领域的新手，所以请帮助我以更好的方式清楚地理解事情。下面给出了我的工作查询，并提到了对我来说没有意义的行。谢谢。

0 sdebit_amount，

0 scredit_amount，

0 scredit_amount，

SELECT 
MONTH AS month,
ROUND(IFNULL(scredit_amount,0),2) AS purchase,
ROUND(IFNULL(sdebit_amount,0),2) AS cash_paid
FROM 
(SELECT 
MONTHNAME(sc.scredit_date) as month,
IFNULL(SUM(sc.scredit_amount),0) AS scredit_amount,
0 sdebit_amount
FROM
sup_credit sc 
WHERE YEAR(sc.scredit_date) = YEAR(NOW()) 
GROUP BY MONTHNAME(sc.scredit_date) 

union all

SELECT 
MONTHNAME(sd.sdebit_date) as month,
0 scredit_amount,
IFNULL(SUM(sd.sdebit_amount),0) AS sdebit_amount
FROM
sup_debit sd
WHERE YEAR(sd.sdebit_date) = YEAR(NOW()) 
GROUP BY MONTHNAME(sd.sdebit_date)

union all

SELECT
 MONTHNAME(tpr.date) as month,
 0 scredit_amount,
 IFNULL(SUM(tpr.sub_total),0) sdebit_amount
 FROM
 tbpurchase_return tpr WHERE tpr.method='CREDIT'
 AND YEAR(tpr.date) = YEAR(NOW())
 GROUP BY MONTH(tpr.date)
 ) ABC
 group by month

Answer 1

该查询使用 UNION 将包含贷方记录的表和包含借方记录的表组合起来。贷方表没有 debit_amount 列，借方表也没有 credit_amount 表。组合表需要这两列。因此，当查询从 sup_credit 表中进行选择时，会添加一个带有 0 的 debit_amount 列，并添加一个 credit_amount 列当它从 sup_debit 表中选择时，会显示 0。

然后外部查询将所有内容相加。