SQL：有什麼方法可以在更新聲明中使用Order By？

Question

我需要僅在數據庫中更新一條記錄，並將其分配給用戶。這是我在做什麼：

UPDATE TOP (1) books SET assigneduser = 1 
WHERE bookstatus = 7 
AND ((assigneduser is null) or (assigneduser = 1)); 

我也有一個字段名爲bookname，我寧願命令，但更新似乎並不支持它。

另外請注意，我將使用該軟件同時有50個用戶，所以我需要確保只有一個用戶被分配一本書。否則，我會首先運行一個選擇，然後在頂部記錄上運行更新。

謝謝。

Answer 1

由於update沒有返回任何數據，'order by'對結果集進行排序，所以ORDER BY沒有任何用處，您無法完成所尋找的任務。

如果你想只分配一本書，你可以使用更新頂部1聯接從你設置。

UPDATE b0 SET assigneduser = 1 
FROM b0 
    INNER JOIN 
(SELECT top 1 id FROM books 
WHERE user = 1 OR user is null 
AND status =7) b1 ON b1.id = b0.id 

或可能不太神祕

UPDATE user SET assigneduser = 1 
WHERE id IN 
(SELECT top 1 id FROM books 
WHERE user = 1 OR user is null 
AND status =7) b1 ON b1.id = b0.id 

無論是戰略穩健關於併發事務語義的問題。

Answer 2

UPDATE B 
SET assigneduser = 1 
FROM books B 
WHERE bookstatus = 7 
AND ((assigneduser is null) or (assigneduser = 1)) 
and bookid = (select min(bookid) from books where assigneuser is null) 

我假設你有一個ID列bookid。

這讓你沒有指定用戶最低書ID，並加入回書給你一個記錄（如B的一部分），你可以再更新。

Answer 3

看http://msdn.microsoft.com/en-us/library/ms177523.aspx。

如果必須使用TOP以一種有意義的年表應用更新，你必須與ORDER子查詢中語句中使用TOP BY。以下示例更新了最早僱用日期的10名員工的休假時間。

UPDATE HumanResources.Employee 
SET VacationHours = VacationHours + 8 
FROM (SELECT TOP 10 BusinessEntityID FROM HumanResources.Employee 
    ORDER BY HireDate ASC) AS th 
WHERE HumanResources.Employee.BusinessEntityID = th.BusinessEntityID; 
GO 

Answer 4

如果您只想更新數據庫中的一行，最好的辦法是找出它的主鍵並使用它。你可以這樣

UPDATE books SET assigneduser = 1 
WHERE BOOKID 
= (SELECT top 1 BOOKID FROM books where 
bookstatus = 7 
AND ((assigneduser is null) or (assigneduser = 1))); 

Answer 5

你必須首先選擇所需的記錄做到這一點，那麼更新：

update books 
    set assigneduser = 1 
where BookPrimaryKeyField = (
    SELETE TOP 1 BookPrimaryKeyField 
    from books 
    WHERE bookstatus = 7 
    AND ((assigneduser is null) or (assigneduser = 1)); 
) 

Answer 6

它可能會更容易打入2個

DECLARE @bookid as INT 

    SELECT TOP (1) @bookid = id FROM books 
    WHERE bookstatus = 7 
    AND ((assigneduser is null) or (assigneduser = 1)) 
    ORDER BY bookname 

    UPDATE books SET assigneduser = 1 
    WHERE id = @bookid 

Answer 7

這

有一種方法可以通過使用子查詢來避開它，如下所示：

UPDATE books SET assigneduser=1 
AND ((assigneduser is null) or (assigneduser = 1)) 
AND bookname in (SELECT TOP 1 bookname FROM Table ORDER BY bookname DESC)