与 group by (Sum) 的多个联接

Question

当我使用多个 JOIN 时，我希望得到连接表中某些列的总和。

SELECT 
    A.*, 
    SUM(C.purchase_price) AS purcchase_total, 
    SUM(D.sales_price) AS sales_total, 
    B.user_name
FROM
    PROJECT AS A 
LEFT JOIN 
    USER AS B ON A.user_idx = B.user_idx 
LEFT JOIN 
    PURCHASE AS C ON A.project_idx = C.project_idx
LEFT JOIN 
    SALES AS D ON A.project_idx = D.project_idx
GROUP BY 
    ????

Answer 1

您需要按如下方式使用子查询：

SELECT A.project_idx,
       a.project_name, 
       A.project_category,
       sum(C.purchase_price) AS purcchase_total, 
       sum(D.sales_price) as sales_total,
       B.user_name
FROM PROJECT AS A 
LEFT JOIN USER AS B ON A.user_idx = B.user_idx 
LEFT JOIN (select project_idx, sum(purchase_price) as purchase_price 
             from PURCHASE group by project_idx ) AS C ON A.project_idx = C.project_idx
LEFT JOIN (select project_idx, sum(sale_price) as sale_price 
             from SALES group by project_idx) AS D ON A.project_idx = D.project_idx

我不确定，但您可以将project 的inner join 与user 一起使用，而不是left join。

Answer 2

SELECT A.project_idx,
       a.project_name, 
       A.project_category,
       purcchase_total, 
       sales_total,
       B.user_name
FROM PROJECT AS A 
LEFT JOIN USER AS B ON A.user_idx = B.user_idx 
LEFT JOIN (select project_idx, sum(purchase_price) as purchase_total 
             from PURCHASE group by project_idx ) AS C ON A.project_idx = C.project_idx
LEFT JOIN (select project_idx, sum(sale_price) as sale_total 
             from SALES group by project_idx) AS D ON A.project_idx = D.project_idx

这在 MS-SQL Server 上工作正常。 感谢大力水手

Answer 3

您正在尝试聚合两个不相关的维度，这会导致所有计算失败。

相关子查询是一种替代方法：

SELECT p.*, 
       (SELECT SUM(pu.purchase_price)
        FROM PURCHASE pu
        WHERE p.project_idx = pu.project_idx
       ) as purchase_total,
       (SELECT SUM(s.sales_price)
        FROM SALES s
        WHERE p.project_idx = s.project_idx
       ) as sales_total, 
       u.user_name
FROM PROJECT p LEFT JOIN
     USER u
     ON p.user_idx = u.user_idx ;

请注意，这使用了有意义的表别名，因此查询更易于阅读。任意字母实际上并不比使用整个表名更好（甚至可能更糟）。

相关子查询也避免了外部聚合——让您从第一个表中选择所有列，这正是您想要的。如果使用正确的索引，它们通常也有更好的性能。