SQLServer从子查询中获取前1行

Question

在一个巨大的产品查询中,我试图获得每个元素的最后购买价格.在我的第一种方法中,我在购买表上添加了一个子查询,按日期后代排序,只获得第一行,所以我确保我得到了最新的一行.但它没有表现出任何价值.现在我再次看到它,这是合乎逻辑的,因为子查询仍然没有对产品的限制,然后列出所有购买并获得最新的购买,而不必是当前正在处理的产品主要查询.所以什么都不回报

这是非常简化的代码:

SELECT P.ID, P.Description, P... blah, blah
FROM Products
LEFT JOIN (
    SELECT TOP 1 B.Product,B.Date,B.Price --Can't take out TOP 1 if ORDER BY
    FROM Buys B
    --WHERE... Can't select by P.Product because doesn't exist in that context
    ORDER BY B.Date DESC, B.ID DESC
) BUY ON BUY.Product=P.ID
WHERE (Some product family and kind restrictions, etc, so it processes a big amount of products)

我在主select stmt中考虑了一个嵌入式查询,但由于我需要几个值,因此它意味着对每个值进行查询,这很丑陋.

有没有办法做到这一点,避免臭名昭着的LOOP？谁知道好？

Answer 1

你正走在使用的道路上outer apply,所以让我们继续:

SELECT P.ID, P.Description, P... blah, blah
FROM Products p OUTER APPLY
     (SELECT TOP 1 B.Product,B.Date,B.Price --Can't take out TOP 1 if ORDER BY
      FROM Buys b
    --WHERE... Can't select by P.Product because doesn't exist in that context
      WHERE b.Product = P.ID
      ORDER BY B.Date DESC, B.ID DESC
     ) buy
WHERE (Some product family and kind restrictions, etc, so it processes a big amount of products)

在这种情况下,您可以将其apply作为可以返回多列的相关子查询.在其他数据库中,这称为"横向连接".

Answer 2

似乎是一个不错的候选人OUTER APPLY。你需要一些类似的东西..

SELECT P.ID, P.Description, P... blah, blah
FROM Products P
OUTER APPLY (
    SELECT TOP 1 B.Product,B.Date,B.Price 
    FROM Buys B
    WHERE B.ProductID = P.ID
    ORDER BY B.Date DESC, B.ID DESC
) a