SQL中暗含的連接表中的空值

Question

在連接具有一對多關係的兩個表中，我想要一個結果顯示多個表中的所有值，其中null值在一個表中。

下面是一個例子：

tbl_Platform：PriKey = PlatformID

PlatformID PlatformDesc 
1 Application 
2 Cloud 
3 Storage 
4 Backup 

tbl_Missed：PriKey =客戶id +周+ PlatformID

CustomerID Week Missed  Platform ID 
49   1  2017-05-19 1 

輸出期望：

CustomerID Week Missed PlatformDesc 
49 1 2017-05-19 Application 
49 1 null Cloud 
49 1 null Storage 
49 1 null Backup 

我已經能夠使用交叉連接如下來最接近的：

SELECT   
    dbo.tbl_Missed.CustomerID, 
    dbo.tbl_Missed.Week, 
    dbo.tbl_Missed.Missed, 
    dbo.tbl_Platform.PlatformDesc 
FROM 
    dbo.tbl_Platform 
CROSS JOIN dbo.tbl_MissedSPT 

這給了我：

49 1 2017-05-19 Application 
49 1 2017-05-19 Cloud 
49 1 2017-05-19 Storage 
49 1 2017-05-19 Backup 

Answer 1

看來，如果你想利用價值屬性missed如果在platformID和null中存在匹配，則返回true。嘗試以下（希望沒有錯別字）：

SELECT   
    dbo.tbl_Missed.CustomerID, 
    dbo.tbl_Missed.week, 
    CASE WHEN dbo.tbl_Missed.PlatformID = dbo.tbl_Platform.PlatformID 
       THEN dbo.tbl_Missed.missed 
       ELSE NULL 
    END as missed, 
    dbo.tbl_Platform.PlatformDesc 
FROM dbo.tbl_Platform CROSS JOIN dbo.tbl_MissedSPT