如何用聯接替換子查詢？

（所有表都列上創建和LastEdited作爲時間戳）

table Process 
- ID 
- Title 

table ProcessHistory 
- ID 
- ProcessID 
- HistoryID 

table History 
- ID 
- Title (new, open, closed etc.)

當我試圖讓進程列表與上次狀態稱號的cols我做的：

SELECT DISTINCT Process.*, History.Title AS HistoryTitle, History.ID AS HistoryID 
FROM `Process` 
LEFT JOIN ProcessHistory AS ProcessHistory ON Process.ID=ProcessHistory.ProcessID 
LEFT JOIN History AS History ON HistoryID=ProcessHistory.HistoryID 
WHERE History.ID = (
    SELECT HistoryID FROM ProcessHistory 
    WHERE ProcessID=Process.ID 
    ORDER BY ProcessHistory.ID DESC LIMIT 1 
) 
GROUP BY Process.ID 
ORDER BY Process.ID DESC LIMIT 0, 100

當我嘗試獲取按特定狀態過濾的列表（其中最新HistoryID爲1 - 「所有打開的過程」）

SELECT DISTINCT Process.*, History.Title AS HistoryTitle, History.ID AS HistoryID 
FROM `Process` 
LEFT JOIN ProcessHistory AS ProcessHistory ON Process.ID=ProcessHistory.ProcessID 
LEFT JOIN History AS History ON HistoryID=ProcessHistory.HistoryID 
WHERE History.ID=(
    SELECT HistoryID FROM ProcessHistory 
    WHERE HistoryID =1 
    AND ProcessID=Process.ID ORDER BY ProcessHistory.ID DESC LIMIT 1 
) 
GROUP BY Process.ID 
ORDER BY Process.ID DESC LIMIT 0, 100

由於性能原因，我想擺脫這些子查詢？我如何替換子查詢？

在此先感謝！

來源

2016-07-15 derRobert

由您創建和加載的sqlfiddle將幫助我們幫助您。注意：加載並不意味着完全加載。這可能意味着每個表格有5到10行數據。 – Drew

這裏的sql小提琴： http://sqlfiddle.com/#!9/3d9001/1 – derRobert

它真的是性能瓶頸還是這是不成熟的，甚至可能是非優化的優化？ – e4c5

您仍然需要再作選擇濾除歷史過程的其餘部分，但你應該使用derived table，而不是一個子查詢，這樣sqlfiddle：

SELECT Process.*, History.Title AS HistoryTitle 
FROM Process 
JOIN (
    SELECT ProcessID, max(HistoryID) as HistoryID 
    FROM ProcessHistory 
    GROUP BY ProcessID 
) PH ON PH.ProcessID = Process.ID 
JOIN History ON History.ID = PH.HistoryID 
ORDER BY Process.ID DESC LIMIT 0, 100

由於人士Himanshu帕特爾指出，，「查詢按特定狀態篩選的列表（最新的HistoryID爲1 - 」所有打開的進程「）」的查詢不會產生所需的效果。它將簡單地返回歷史ID爲1的所有進程。請參閱此sqlfiddle。

相反，你要使用派生表來獲得這些最新的工藝歷史的ID，用流程加入他們，和歷史上的ID過濾這樣sqlfiddle：

SELECT DISTINCT Process.*, History.Title AS HistoryTitle 
FROM Process 
JOIN (
    SELECT ProcessID, max(HistoryID) as HistoryID 
    FROM ProcessHistory 
    GROUP BY ProcessID 
) PH ON PH.ProcessID = Process.ID 
JOIN History ON History.ID = PH.HistoryID 
WHERE PH.HistoryID = 1 
ORDER BY Process.ID DESC LIMIT 0, 100

另一種方法是創建一個視圖，將ProcessHistory表過濾到每個進程的最新歷史記錄並加入。 YMMV，但在某些情況下，可以通過這種方式改善性能。

來源

2016-07-22 18:29:08 Will

查詢您張貼不給正確的結果爲每ORDER BY ProcessHistory.ID DESC LIMIT 1

嘗試下面的查詢按查詢

SELECT DISTINCT p.*, h.Title AS HistoryTitle,h.ID AS HistoryID 
FROM 
Process p JOIN ProcessHistory ph ON p.ID=ph.ProcessID and ph.HistoryID=1 
JOIN History h ON h.ID=ph.HistoryID 
GROUP BY p.ID 
ORDER BY p.ID DESC LIMIT 0, 100;

這裏的sql小提琴的結果：
sql fiddle link

如果希望其他結果然後評論。

來源

2016-07-18 08:02:33

SELECT * 
FROM Process AS p 
LEFT JOIN (
    SELECT ph.ProcessID, ph.HistoryID, h.Title AS HistoryTitle 
    FROM ProcessHistory AS ph 
    JOIN History AS h ON ph.HistoryID = h.ID 
    WHERE h.ID = 1 
) AS phh ON p.ID = phh.ProcessID 
ORDER BY p.ID DESC LIMIT 100

來源

2016-07-19 04:16:56 SIDU

由於實際子查詢，用於獲取有關每一個「過程」，「最後」行，你既不能把它轉換爲連接，也可以使用它作爲一個單獨的查詢設置一個會話變量。

來源

2016-07-21 18:50:48 Uueerdo

如何用聯接替換子查詢？

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