Mysql 多连接查询

Question

我有下表，希望得到如下结果

Table po
ID | Date
1  | 20-Jun-2016

Table podetails
ID | poid | itemcode | quantity
1  | 1    | SOAP123  | 100

Table poreceived
ID | poid | itemcode | quantity
1  | 1    | SOAP123  | 20
2  | 1    | SOAP123  | 60

结果应该是:

PO | Date        | itemcode | quantity
1  | 20-Jun-2016 | SOAP123  | 80

我所做的是:

SELECT 
    po.id, podetails.itemcode, poreceived.quantity
FROM
    po
        LEFT JOIN
    podetails ON podetails.poid = po.id
        LEFT JOIN
    poreceived ON poreceived.poid = podetails.poid

但是结果并不是我所期望的。

如有任何帮助，我们将不胜感激。

Answer 1

看起来您需要通过 poid 对 poreceived 表进行聚合。您可以为此使用子查询:

SELECT po.id, COALESCE(podetails.itemcode, 'NA'), COALESCE(t.quantity, 0)
FROM po
LEFT JOIN podetails
    ON podetails.poid = po.id
LEFT JOIN
(
    SELECT poid, itemcode, SUM(quantity) AS quantity
    FROM poreceived
    GROUP BY poid, itemcode
) t
    ON t.poid = podetails.poid AND t.itemcode = podetails.itemcode

除了聚合问题之外，您的查询策略看起来是正确的。我还在 podetails 和 poreceived 表中添加了 COALESCE ，以防 po 中的 id 不匹配任何事情。