mysql内部加入群组

Question

我正在使用以下查询来获取每个批次的正确结果。例如，如果我会喜欢看总发票批2010 ...

SELECT COALESCE(sum(i.amount),0) AS amount, 
    COALESCE(sum(i.discount),0) AS discount, 
    COALESCE(sum(i.amount) - sum(i.discount),0) AS netpay, 
    b.name AS batch 
FROM fm_batches b 
    INNER JOIN fm_invoices i 
    LEFT JOIN fm_students s ON i.student_id = s.id 
GROUP BY b.name 

而且其输出结果如下......

| amount | discount | netpay | batch | 
+--------+----------+----------+-------+ 
| 2500 | 500  | 2000  | 2011 | 
+--------+----------+----------+-------+ 
| 2500 | 500  | 2000  | 2010 | 
+--------+----------+----------+-------+ 
| 2500 | 500  | 2000  | 2009 | 
+--------+----------+----------+-------+ 
| 2500 | 500  | 2000  | 2008 | 
+--------+----------+----------+-------+ 

我相信，我在做什么我的查询错误，因为它提供了错误的结果。它应该返回0，如果没有找到批2010年。谢谢。

Answer 1

所以，你需要这样的东西：

SELECT COALESCE(sum(i.amount),0) AS amount, 
    COALESCE(sum(i.discount),0) AS discount, 
    COALESCE(sum(i.amount)-sum(i.discount),0) AS netpay, 
    b.name AS batch 
FROM batches b 
LEFT JOIN subscribers s on s.bacth_id = b.id 
LEFT JOIN invoices i on i.subs_id = s.id 
GROUP BY b.name 

（猜测用户和批次之间的关系）。