從多行中選擇單行 - Teradata

Question

我有以下要求來選擇最新狀態。

表1：

表2：

結果預期：

下面是我們孃家的邏輯d。

SELECT 
     Table1.ID, 
     ,CASE WHEN (Table1.hub=Table2.hub) THEN Table2.Status ELSE NULL END AS Original_Status 
     ,CASE WHEN (Table1.hub<>Table2.hub AND Table2.Status like 'Found%') THEN Table2.hub ELSE NULL END AS Derived_Hub 
     ,CASE WHEN (Table1.hub<>Table2.hub AND Table2.Status like 'Found%') THEN Table2.Status ELSE NULL END AS Derived_Status 
from 
    Table1 
Join Table2 
    ON (Table1.ID=Table2.ID) 

有了這段代碼，我得到了3行。如果我在上面的case語句中使用max，我得到Hub3而不是Hub2。

請問我可以讓我知道如何將所有東西合併爲一行。請注意，即使有2個已發現狀態的集線器，我也想從表2中顯示最新發現的狀態。

讓我知道你是否需要任何進一步的信息。

Answer 1

我認爲你最好的選擇是加入表2兩次。第一次，很像你在這裏做的，但在id和hub上都使用LEFT OUTER JOIN。然後第二次對錶2的衍生版本，你只有當時間戳降序排序選擇頂部1記錄：

SELECT 
    t1.id, 
    t1.hub as "Original Hub" 
    t2.status as "Original Hub Status", 
    t3.hub as "Found Hub", 
    t3.status as "Found Hub Status" 
FROM 
    Table1 t1 
    LEFT OUTER JOIN Table2 t2 ON 
     t1.id = t2.id AND 
     t1.hub = t2.hub 
    LEFT OUTER JOIN 
     (
      --Select a hub with the same id, that doesn't share the same hub number 
      --Only choose the top record when sorted by timestamp in descending order   
      SELECT TOP 1 id, hub, status 
      FROM table2 
      WHERE t1.hub <> table2.hub 
      ORDER BY TimeStamp Desc 
     ) t3 ON 
     t1.id = t3.id 

有別名t3被引用在t1.hub值使用相關子查詢的子查詢它是WHERE語句。

Answer 2

在TD14.10中，您可以利用LAST_VALUE訪問「最後」行中的數據。該CASE S是根據您的查詢：

SELECT 
    t2.ID, 
    CASE WHEN t1.Hub = t2.Hub THEN t2.Hub END AS Original_Hub, 
    CASE WHEN t1.Hub = t2.Hub THEN t2.Status END AS Original_Status, 
    -- get the last Hub 
    LAST_VALUE(CASE WHEN t1.Hub <> t2.Hub AND Table2.Status like 'Found%' 
        THEN t2.Hub 
       END) 
    OVER (PARTITION BY t1.ID 
     ORDER BY CASE WHEN t1.Hub = t2.Hub THEN 0 ELSE 1 END, t2.TS 
     ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS Derived_Hub, 
    -- get the last Status 
    LAST_VALUE(CASE WHEN t1.Hub <> t2.Hub AND Table2.Status like 'Found%' 
        THEN t2.Status 
       END) 
    OVER (PARTITION BY t1.ID 
     ORDER BY CASE WHEN t1.Hub = t2.Hub THEN 0 ELSE 1 END, t2.TS 
     ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS Derived_Status 
FROM 
    Table1 AS t1 
JOIN Table2 AS t2 
    ON (t1.ID=t2.ID) 
QUALIFY 
    ROW_NUMBER() -- return the 1st row only 
    OVER (PARTITION BY t1.ID 
     ORDER BY CASE WHEN t1.Hub = t2.Hub THEN 0 ELSE 1 END, t2.TS) = 1 

EXPLAIN都應該OVER合併成一個單一的STATS-一步，因爲它們使用相同的PARTITION BY和ORDER BY。