与 group by (Sum) 的多个联接

Question

When I using multiple JOIN, I hope to get the sum of some column in joined tables.

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

You need to use subquery as follows:

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

I am not sure but you can use inner join of project with user instead of 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

This is working correctly on MS-SQL Server. Thanks to Popeye

Answer 3

You are attempting to aggregate over two unrelated dimensions, and that throws off all the calculations.

Correlated subqueries are an alternative:

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 ;

Note that this uses meaningful table aliases so the query is easier to read. Arbitrary letters are really no better (and perhaps worse) than using the entire table name.

Correlated subqueries avoid the outer aggregation as well -- and let you select all the columns from the first table, which is what you want. They also often have better performance with the right indexes.