SQL SELECT WHERE故障很有幫助

Question

任何想法，我有兩個表父和子（相關這樣的PK/FK GUID）

的孩子有一個時間戳（記錄創建日期/時間）。

我想要做的是隻獲取最新的子記錄和父記錄，爲每個父記錄。

SELECT 
    dbo_Parents.ParentName, 
    dbo_ChildEntry.CountPropertys, 
    dbo_ChildEntry.DateTimeStamp 
FROM 
    dbo_Parents INNER JOIN dbo_ChildEntry 
ON 
    dbo_Parents.ParentID = dbo_ChildEntry.ParentID 
WHERE 
    ([WHAT SHOULD BE HERE?])) 

Answer 1

假設你想要的最新的條目，你必須使用TOP 1和秩序。澄清後

SELECT TOP 1 
dbo_Parents.ParentName, 
dbo_ChildEntry.CountPropertys, 
dbo_ChildEntry.DateTimeStamp 
FROM dbo_Parents 
INNER JOIN dbo_ChildEntry ON dbo_Parents.ParentID = dbo_ChildEntry.ParentID 
ORDER BY dbo_ChildEntry.DateTimeStamp desc 

編輯： 「最近的子記錄與父記錄，爲每個父記錄」：

WHERE dbo_ChildEntry.DateTimeStamp = 
     (Select Max(dbo_ChildEntry.DateTimeStamp) 
       from dbo_ChildEntry 
       where dbo_Parents.ParentID = dbo_ChildEntry.ParentId) 

Answer 2

WHERE dbo.ChildEntry.DateTimeStamp = (Select Max(c.DateTimeStamp) 
             from dbo.ChildEntry c 
             where dbo_Parents.ParentID = c.ParentId) 

Answer 3

嘗試

SELECT  dbo_Parents.ParentName,  dbo_ChildEntry.CountPropertys,  
max(dbo_ChildEntry.DateTimeStamp) 
FROM  dbo_Parents 
INNER JOIN dbo_ChildEntry ON dbo_Parents.ParentID = dbo_ChildEntry.ParentID 
group by dbo_Parents.ParentName,  dbo_ChildEntry.CountPropertys 

Answer 4

select p.ParentName, 
    c.CountPropertys, 
    c.DateTimeStamp 
from (
    select max(DateTimeStamp) as DateTimeStamp, ParentID 
    from dbo_ChildEntry 
    group by ParentID) cm 
inner join dbo_ChildEntry c on cm.DateTimeStamp = c.DateTimeStamp and cm.ParentID = c.ParentID 
inner join dbo_Parents p on c.ParentID = p.ParentID 

Answer 5

由於你澄清你想每個父母一行，試試這個。對於每一行，它都會搜索具有較晚時間戳的子行。在WHERE子句中，它過濾沒有「下一個」子項的行。

SELECT 
    dbo_Parents.ParentName, 
    cur.CountPropertys, 
    cur.DateTimeStamp 
FROM dbo_Parents 
INNER JOIN dbo_ChildEntry cur 
    ON dbo_Parents.ParentID = cur.ParentID 
LEFT JOIN dbo_ChildEntry next 
    ON dbo_Parents.ParentID = next.ParentID 
    AND next.DateTimeStamp > cur.DateTimeStamp 
WHERE 
    next.DateTimeStamp is null