Question

SELECT sq.type, sq.cust, MAX(sq.sum)
FROM (
    SELECT C.custid AS cust, P.ptype AS type, SUM(D.qty) AS sum
    FROM Customers C, Orders O, Details D, Products P
    WHERE C.custid = O.ocust and D.ordid = O.ordid and P.pcode = D.pcode    
    GROUP BY cust, type
) as sq
GROUP BY sq.type
;

你好朋友

我的问题是我希望能够通过SQL中的group by函数选择sq.cust和sq.type。当使用MAX聚合时，无论如何它仅应返回1个元组，其中包括sq.type和sq.cust，那么为什么我无法通过这种方式访问它？有什么方法可以返回该产品类型（sq.type）的最大平方和的客户ID（sq.cust）？

谢谢！

Answer 1

从不在FROM子句中使用逗号。始终使用正确的，明确的，标准 JOIN语法。

也就是说，您可以在Postgres中使用distinct on：

SELECT DISTINCT ON (ptype) C.custid as cust, P.ptype AS type, SUM(D.qty) AS sum
FROM Customers C JOIN
     Orders O
     ON C.custid = O.ocust JOIN
     Details D
     ON D.ordid = O.ordid JOIN
     Products P
     ON P.pcode = D.pcode    
GROUP BY cust, ptype
ORDER BY ptype, SUM(d.QTY) DESC

Answer 2

尽管Pg特定，但Gordon Linoff的解决方案很棒。为了使其更易于移植，可以使用窗口功能，该功能现在在大多数主要的RDBMS中都可用。

例如，在SQL Server中：

select C.custid as cust, P.ptype, sum(qty) as sum_qty
    , FIRST_VALUE(sum(qty)) OVER (PARTITION BY ptype ORDER BY SUM(qty) DESC) as max_sum_qty_for_ptype
FROM Customers C JOIN
     Orders O
     ON C.custid = O.ocust JOIN
     Details D
     ON D.ordid = O.ordid JOIN
     Products P
     ON P.pcode = D.pcode

GROUP BY cust, ptype
ORDER BY ptype, SUM(QTY) DESC

很明显，您可以删除Sum_qty列

在SQL中将GROUP BY与MAX聚合一起使用，并在GROUP BY语句中未选择的其他列

2 个答案: