mysql內部加入羣組

Question

我正在使用以下查詢來獲取每個批次的正確結果。例如，如果我會喜歡看總髮票批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 

而且其輸出結果如下......

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

我相信，我在做什麼我的查詢錯誤，因爲它提供了錯誤的結果。它應該返回0，如果沒有找到批2010年。謝謝。

Answer 1

所以，你需要這樣的東西：

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 

（猜測用戶和批次之間的關係）。