SQL求和加入

Question

我有表A，B，c和d，其中：

There are 0 or more b rows for each a row 
There are 0 or more c rows for each a row 
There are 0 or more d rows for each a row 

如果我試着像下面這樣的查詢：

SELECT a.id, SUM(b.debit), SUM(c.credit), SUM(d.other) 
FROM a 
LEFT JOIN b on a.id = b.a_id 
LEFT JOIN c on a.id = c.a_id 
LEFT JOIN d on a.id = d.a_id 
GROUP BY a.id 

我注意到，我有創造了笛卡兒的產品，因此我的款項不正確（太大）。

我看到還有其他的問題和答案，但是我仍然沒有把握如何在單個查詢中完成我想要做的事情。是否有可能在SQL中編寫一個聚合了以下所有數據的查詢：

SELECT a.id, SUM(b.debit) 
FROM a 
LEFT JOIN b on a.id = b.a_id 
GROUP BY a.id 

SELECT a.id, SUM(c.credit) 
FROM a 
LEFT JOIN c on a.id = c.a_id 
GROUP BY a.id 

SELECT a.id, SUM(d.other) 
FROM a 
LEFT JOIN d on a.id = d.a_id 
GROUP BY a.id 

在單個查詢中？

Answer 1

SELECT a.ID, debit, credit, other 
FROM a 
LEFT JOIN (SELECT a_id, SUM(b.debit) as debit 
      FROM b 
      GROUP BY a_id) b ON a.ID = b.a_id 
LEFT JOIN (SELECT a_id, SUM(b.credit) as credit 
      FROM c 
      GROUP BY a_id) c ON a.ID = c.a_id 
LEFT JOIN (SELECT a_id, SUM(b.other) as other 
      FROM d 
      GROUP BY a_id) d ON a.ID = d.a_id 

Answer 2

還可以與相關子查詢做到：

SELECT a.id 
    , (SELECT SUM(debit) FROM b WHERE a.id = b.a_id) 
    , (SELECT SUM(credit) FROM c WHERE a.id = c.a_id) 
    , (SELECT SUM(other) FROM d WHERE a.id = d.a_id) 
FROM a 

Answer 3

你的分析是正確的。無關JOIN創建笛卡爾產品。

你必須單獨完成這筆款項，然後做最後的補充。這是一個查詢可行的，你有幾個選項：然後SELECT a.id, b_sum + c_sum + d_sum

UNION ALL爲您推薦SELECT a.id, (SELECT SUM(b.debit) FROM b WHERE b.a_id = a.id) + ...

CROSS APPLY有類似查詢作爲第一彈：

• 子請求在你的SELECT最重要的是外層SUM和GROUP BY。
• LEFT JOIN與以上類似的子查詢。

而且可能更多...各種解決方案的性能可能會略有不同，具體取決於您想要選擇的A中有多少行。