使用 WITH 子句的结果执行 JOIN

Question

我有一个查询已经使用了一个连接，称之为 query1。我想将 query1 加入 WITH 子句的结果。我不知道如何合并两个数据集。

查询一：

SELECT 
    p.NetObjectID 
    , n.Caption, n.ObjectSubType
FROM Pollers p
LEFT JOIN NodesData n ON n.NodeID = p.NetObjectID

下一个查询是 WITH 子句。我不知道如何在没有 WITH 的情况下获得结果，因为我需要执行this post 中概述的操作。

查询 2：

WITH ranked_DateStamp AS 
(
    SELECT c.NodeID, c.DateTime
        , ROW_NUMBER() OVER (PARTITION BY NodeID ORDER BY DateTime DESC) AS rn
    FROM CPULoad AS c
)
SELECT *
FROM ranked_DateStamp
WHERE rn = 1;

我以为我可以在ranked_DateStamp.NodeID = p.NodeID 上加入ranked_DateStamp，但它不允许。

Answer 1

实际上，我会建议一种不同的方法——横向连接：

SELECT p.NetObjectID, n.Caption, n.ObjectSubType,c.DateTime
FROM Pollers p LEFT JOIN
     NodesData n
     ON n.NodeID = p.NetObjectID OUTER APPLY
     (SELECT TOP (1) NodeID, DateTime, 
        ROW_NUMBER() OVER (PARTITION BY NodeID ORDER BY DateTime DESC) AS rn
      FROM CPULoad c
      WHERE c.NodeId = p.NetObjectID
      ORDER BY DateTime DESC
     ) c;

横向连接非常强大，值得学习。它们通常也比 ROW_NUMBER() 方法快一点。

Answer 2

您真的在这里不需要with 子句，您可以使用子查询。我只是将其表述为：

SELECT 
    p.NetObjectID, 
    n.Caption, 
    n.ObjectSubType,
    c.DateTime
FROM Pollers p
LEFT JOIN NodesData n ON n.NodeID = p.NetObjectID
LEFT JOIN (
    SELECT 
        NodeID, 
        DateTime, 
        ROW_NUMBER() OVER (PARTITION BY NodeID ORDER BY DateTime DESC) AS rn
    FROM CPULoad AS c
) c ON c.rn = 1 and c.NodeID = p.NetObjectID

但是，如果您要使用公用表表达式，则如下所示：

WITH ranked_DateStamp AS (
    SELECT 
        NodeID, 
        DateTime, 
        ROW_NUMBER() OVER (PARTITION BY NodeID ORDER BY DateTime DESC) AS rn
    FROM CPULoad AS c
)
SELECT 
    p.NetObjectID, 
    n.Caption, 
    n.ObjectSubType,
    c.DateTime
FROM Pollers p
LEFT JOIN NodesData n ON n.NodeID = p.NetObjectID
LEFT JOIN ranked_DateStamp c ON c.rn = 1 and c.NodeID = p.NetObjectID

实际上，横向连接的性能可能同样好，甚至更好：

SELECT 
    p.NetObjectID, 
    n.Caption, 
    n.ObjectSubType,
    c.DateTime
FROM Pollers p
LEFT JOIN NodesData n ON n.NodeID = p.NetObjectID
OUTER APPLY (SELECT TOP (1) * FROM CPULOad c WHERE c.NodeID = p.NetObjectID ORDER BY DateTime DESC) c