從兩個表中獲取MAX（date）的值

Question

我有兩個表。

MainTable: 
MainID | LastValue | LastReadingDate 
1  | 234  | 01.01.2012 
2  | 534  | 03.02.2012 

Readings: 
MainID | ValueRead | ReadingDate 
1  | 123  | 03.02.2012 
1  | 488  | 04.03.2012 
2  | 324  | 03.02.2012 
2  | 683  | 05.04.2012 

我想

SELECT MainTable.MainID, MainTable.LastValue, MainTable.LastReadingDate, (SELECT ValueRead, MAX(ReadingDate) 
FROM Readings 
WHERE Readings.MainID=MainTable.MainID ORDER BY ValueRead) 

換句話說，我想目前LastValue和LastReadingDate從MainTable沿側ValueRead從讀數最近ReadingDate。

Answer 1

以下是您可以使用的查詢。它將顯示所有MainTable條目，包括那些還沒有「Reading」條目的條目。如果你不想這樣做，請將LEFT JOIN更改爲INNER JOIN。

WITH LastReads AS (

SELECT ROW_NUMBER() OVER (PARTITION BY MainID ORDER BY ReadingDate DESC) AS ReadingNumber, 
MainID, 
ValueRead, 
ReadingDate 

FROM Readings 

) 

SELECT M.MainID, M.LastValue, M.LastReadingDate, R.ValueRead, R.ReadingDate 
FROM MainTable M 
LEFT OUTER JOIN LastReads R 
ON M.MainID = R.MainID 
AND R.ReadingNumber = 1 -- Last reading, use 2 or 3 to get the 2nd newest, 3rd newest, etc. 

SQLFiddle鏈接：http://sqlfiddle.com/#!3/16c68/3

有N多的每mainid讀數的另一個鏈接：http://sqlfiddle.com/#!3/16c68/4

Answer 2

沒試過此我自己，但在這裏不用。請嘗試

select max(r.readingdate), max(t.lastvalue), max(t.lastreadingdate) 
from readings r inner join 
(select MainID, LastValue, LastReadingDate 
    from MainTable m 
    where LastReadingDate = 
     (select max(minner.LastReadingDate) 
     from MainTable minner 
     where minner.MainID = m.MainID 
    ) 
) t 
on (r.mainid = t.mainid) 

Answer 3

試試這個：

select M.LastValue, M.LastReadingDate, 
     (select top 1 ValueRead from Readings where MainID=M.MainID order by ReadingDate desc) 
from MainTable M