带有连接的SQL查询，但只想要连接表中的最新记录

Question

我有一个包含引擎（EngineId，Make，Model，SerialNumber等）的表的数据库和引擎上具有Test值的表（TestId，EngineId，值1，值2等）。

我想从引擎表中选择所有记录以及最新的测试值（可能有多个测试记录，但我只想要最新的）。这看起来很简单，但由于某种原因，我无法完全掌握SQL语法。

我可以从类似下面的东西开始，但然后我卡住了。

SELECT e.*, t.Value1, t.Value2 
FROM Engine e 
left join Event ev on e.EngineID = ev.EngineID 
left join Test t on ev.EventID = t.EventID 
WHERE ev.EventType = 'TEST' 

我可以关闭以下内容，但它只返回带有测试数据的记录。我想没有测试数据返回引擎记录：

SELECT e.*, t.Nox, t.CO 
FROM Engine e 
left join Event ev on e.EngineID = ev.EngineID 
left join Test t on ev.EventID = t.EventID 
left join EventType et on ev.EventTypeId = et.EventTypeID 
WHERE Upper(et.Name) = 'TEST' 
    AND ev.EventDate= (SELECT MAX(EventDate) 
         FROM Event ev2 
         left join EventType et2 on ev2.EventTypeID = et2.EventTypeID 
         WHERE ev2.EngineID = e.EngineID AND 
         Upper(et2.Name) = 'TEST') 

我可以亲近下面，但我仍然需要加入以某种方式测试表：

SELECT DISTINCT e.*, om.maxdate 
FROM Engine e 
LEFT JOIN (
    select ev2.EngineID, max(EventDate) as maxdate 
    from Event ev2 
    LEFT JOIN EventType et on ev2.EventTypeID = et.EventTypeID 
    WHERE Upper(et.Name) = 'TEST' 
    group by EngineID 
) om on e.EngineID = om.EngineID 
--LEFT JOIN Test t on om.EventID = t.EventID 

我可以做，如果一个存储过程帮助。

你问了一些示例数据，所以在这里。我想要返回所有的引擎记录以及最新的测试值。

引擎

EngineId Make Model SerialNumber 
    1 Waukesha 5794 C-62124 
    2 Waukesha 5800 F-199374 
    3 NULL  NULL B-200305 
    4 Waukesha 6000 D-999777 
    5 Waukesha 6100 E-877324 
    6 Briggs & Stratton LW300 SL-122375 
    7 Briggs & Stratton LW350 SL-375200 
    8 Briggs & Stratton GS250 LS-988755 
    9 Briggs & Stratton 5794 SN12345 

测试

TestID EventID EngineID TestDate Description Value1 Value2 
1 7 1 03/01/17 12:00 AM Testing 30 50 
9 8 1 01/01/17 12:00 AM Testing 20 20 
17 11 1 12/01/16 12:00 AM Testing 16 15 

Answer 1

可以使用

SELECT TOP 1 e. *,t.Value1,t.Value2 

让高层记录，以确保顶部是最新的，责令表

ORDER BY column1, column2, ... ASC|DESC 

希望这有助于....

Answer 2

如果你意味着对给定的发动机最新的测试活动（基于EVENTDATE），那么它就是：

SELECT e.*, t.Value1, t.Value2 
    FROM Engine e 
    LEFT JOIN Event ev 
     ON e.EngineID = ev.EngineID 
      AND ev.EventType = 'TEST' 
      AND ev.EventDate = (SELECT MAX(ev2.EventDate) 
           FROM Event ev2 
           WHERE ev2.EngineID = e.EngineID 
            AND ev2.EventType = 'TEST') 
    LEFT JOIN Test t 
     ON ev.EventID = t.EventID 

请注意，如果最新的事件有更多的测试记录，所有这些都将被返回，因为它们都共享最新的EventDate。但是这似乎是正确的，因为你没有区分分配给同一事件的多个测试记录，所以它们是平等的。

Answer 3

当你说你想要最新的测试值时，这意味着你将有某种方法来识别最新的测试值。这可以通过检索最新的eventdate来完成。

您将不得不在查询中使用子查询。

SELECT e.*, t.Value1, t.Value2 
FROM Engine e 
left join Event ev on e.EngineID = ev.EngineID 
left join Test t on ev.EventID = t.EventID 
WHERE ev.EventType = 'TEST' 
    AND ev.EventDate= (SELECT MAX(EventDate) 
         FROM Event 
         WHERE Event.EventID = t.EventId 
          AND Event.EngineID = ev.EngineId) 

这应该给你最新的测试结果为事件在发动机

Answer 4

可以使用rank()功能只得到每个引擎的最后一次测试。

;with Tcte as (
SELECT e.* 
, t.Value1 
, t.Value2 
, rank() over (partition by e.EngineID order by t.TestDate desc) r 
FROM Engine e 
left join Event ev on e.EngineID = ev.EngineID 
left join Test t on ev.EventID = t.EventID WHERE ev.EventType = 'TEST') 
SELECT * FROM Tcte WHERE r=1 

Answer 5

试试这个

;with testValues as 
(select ev.EngineID, ev.EventId ,t.TestId,t.Value1, t.Value2 
    ,row_number() over(partition by ev.EventId order by ev.EventDate desc)rowNumber 
    from Event ev 
    left join Test t on ev.EventID = t.EventID 
    WHERE ev.EventType = 'TEST' 
) 

SELECT e.*, v.Value1, v.Value2 
FROM Engine e 
left join testValues v on e.EngineID = v.EngineID and v.rowNumber=1 

或者这

;with testData as 
(
SELECT e.*, t.Value1, t.Value2 
,row_number() over(partition by ev.EventId order by ev.EventDate desc)rowNumber 
FROM Engine e 
left join Event ev on e.EngineID = ev.EngineID and ev.EventType = 'TEST' 
left join Test t on ev.EventID = t.EventID 
) 
select * from testData where rowNumber=1