如果月份匹配并按月分组，如何对两个表中的列求和

Question

目前，我已经实现了使用此查询的单个表来做到这一点：

SELECT EXTRACT(MONTH FROM date) as month, SUM(total) as total FROM invoices         GROUP BY month ORDER BY month ASC

但是现在我很疯狂，试图从两列中返回相同的结果，比如total1和total2，并按月分组，如果其中一个列中一个月内没有发票，则结果应该为0。

表结构和预期结果：

invoices payments
date     date
total    income


month      totalInvoices   totalPayments    
1         10005            8017  
2         756335           5019  
3         541005           8017  
4         34243            8870 

我该如何实现？ 有什么建议么？

Answer 1

您需要查询结构的第三个元素，该元素提供所有相关年/月的完整列表。 这可能是现有的表或子查询，但是整个查询将遵循以下概述的结构：

 CREATE TABLE invoices (`id` int, `invdate` datetime, `invtotal` numeric); 

 INSERT INTO invoices (`id`, `invdate`, `invtotal`) VALUES (1, '2017-01-21 00:00:00', 12.45); 

 CREATE TABLE payments (`id` int, `paydate` datetime, `paytotal` numeric); 

 INSERT INTO payments (`id`, `paydate`, `paytotal`) VALUES (1, '2017-02-21 00:00:00', 12.45); 

 select ym.year, ym.month, inv.invtotal, pay.paytotal from ( SELECT EXTRACT(YEAR FROM invdate) as year , EXTRACT(MONTH FROM invdate) as month FROM invoices UNION SELECT EXTRACT(YEAR FROM paydate) as year , EXTRACT(MONTH FROM paydate) as month FROM payments ) ym left join ( SELECT EXTRACT(YEAR FROM invdate) as year , EXTRACT(MONTH FROM invdate) as month , SUM(invtotal) as invtotal FROM invoices GROUP BY year, month ) inv on ym.year = inv.year and ym.month = inv.month left join ( SELECT EXTRACT(YEAR FROM paydate) as year , EXTRACT(MONTH FROM paydate) as month , SUM(paytotal) as paytotal FROM payments GROUP BY year, month ) pay on ym.year = pay.year and ym.month = pay.month; 

\n 年|  一个月  合计  支付总额\n ----- | ------- | ---------- | -------- |\n 2017 |  1 |  12 |  空 |\n 2017 |  2 |  空 |  12 |\n

在我的示例中，“第三个元素”是子查询ym，但这对于您的实际查询而言可能效率太低，但是它应该帮助服务器确定如何在不同的时间范围内协调数据。

dbfiddle 在这里