LEFT JOIN不会返回ULL值

Question

我有两个表：

AppWindowsEvent：

CREATE TABLE [AppWindowsEvent] 
(
[idAppWindowEvent]  INT IDENTITY(1,1) 
, [idAppWindow]   INT 
, [idEventType]   INT 
, [Order]    INT 
, CONSTRAINT PK_idAppWindowEvent PRIMARY KEY ([idAppWindowEvent]) 
, CONSTRAINT FK_idAppWindowEvent_AppWindow FOREIGN KEY ([idAppWindow]) REFERENCES [AppWindow]([idAppWindow]) 
, CONSTRAINT FK_idAppWindowEvent_EventType FOREIGN KEY ([idEventType]) REFERENCES [EventType]([idEventType]) 
) 

事件：

CREATE TABLE [Event]  
(
[idEvent] [INT] IDENTITY(1,1) NOT NULL 
, [idEventType] [INT] NOT NULL 
, [idEntity] [INT] NOT NULL 
, CONSTRAINT PK_IdEvent PRIMARY KEY([idEvent]) 
, CONSTRAINT [FK_Event_EventType] FOREIGN KEY([idEventType]) REFERENCES [EventType] ([idEventType]) 
) 

当我运行此查询：

SELECT 
* 
FROM 
AppWindowsEvent AWE 
LEFT JOIN Event E ON AWE.idEventType = E.idEventType 
WHERE 
AWE.idMill = 1 
AND AWE.idAppWindow = 1 
ORDER BY 
AWE.[Order] ASC 

结果：不返回空值。

当我运行此

SELECT 
* 
FROM 
AppWindowsEvent AWE 
LEFT JOIN Event E ON AWE.idEventType = E.idEventType 
AND E.[idEntity] = 1234 
WHERE 
AWE.idMill = 1 
AND AWE.idAppWindow = 1 
ORDER BY 
AWE.[Order] ASC 

结果：返回空值。

注意： 我需要整套已经配置好的数据，如果你想要一组特定的事件，可以通过事件表的特定idEntity和结果回报很好，但只适用于idEntity，在我的情况下，我需要所有idEntity。

Answer 1

试试这个

SELECT * 
FROM 
AppWindowsEvent AWE 
LEFT JOIN Event E ON AWE.idEventType = E.idEventType 
WHERE 
AWE.idMill = 1 
AND AWE.idAppWindow = 1 
AND E.[idEntity] = 1234 
ORDER BY 
AWE.[Order] ASC 

或者，如果你不希望出现在第二个表空勇气，你可以使用内部联接，而不是左加入

SELECT * 
FROM 
AppWindowsEvent AWE 
Inner JOIN Event E ON AWE.idEventType = E.idEventType 
AND E.[idEntity] = 1234 
WHERE 
AWE.idMill = 1 
AND AWE.idAppWindow = 1 
ORDER BY 
AWE.[Order] ASC