如何通过连接替换子查询？

Question

如何获得这些子查询的rif？

（所有表都有Created和LastEdited列作为时间戳）

table Process
 - ID
 - Title

table ProcessHistory
 - ID
 - ProcessID
 - HistoryID

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

当我尝试获取具有上一个状态标题的cols的进程列表时，我这样做：

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

当我尝试按特定状态过滤列表时 （最新的HistoryID为1 - “所有打开的进程”）

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

出于性能原因，我想摆脱这些子查询？ 我怎样才能替换子查询？

提前致谢！

Answer 1

根据ORDER BY ProcessHistory.ID DESC LIMIT 1发送的查询结果未按照 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; 发送给出正确的结果 根据您的查询结果尝试以下查询

{{1}}

这里sql小提琴：
sql fiddle link

如果您想要其他结果，请发表评论。

Answer 2

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

Answer 3

由于实际的子查询，用于获取与每个“进程”相关的“最后”行，您既不能将其转换为JOIN，也不能将其用作单独的查询来设置会话变量。

Answer 4

您仍然需要制作另一个选择来过滤掉剩余的流程历史记录，但您应该使用derived table而不是子查询，例如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

正如Himanshu Patel所指出的那样，您的查询得到了一个按特定状态过滤的列表（最新的历史ID是1 - ＆＃34;所有打开的流程＆＃34;）＆＃34;不会产生预期的效果。它只会返回历史ID为1的所有进程。请参阅此sqlfiddle。

相反，您希望使用派生表来获取这些最新的流程历史记录ID，将它们与流程连接，并过滤历史记录ID，例如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

另一种方法是创建一个视图，将ProcessHistory表过滤到每个进程的最新历史记录并加入该过程。 YMMV，但在某些情况下，性能可以通过这种方式得到改善。