有條件的內部聯接

Question

我希望能夠根據表達式的結果內聯兩個表。

我一直在試圖至今：

INNER JOIN CASE WHEN RegT.Type = 1 THEN TimeRegistration ELSE DrivingRegistration AS RReg 
ON 
RReg.RegistreringsId = R.Id 

REGT是加入我做之前這個連接：

INNER JOIN RegistrationTypes AS RegT ON R.RegistrationTypeId = RegT.Id 

此SQL腳本不起作用。

所以總而言之，如果Type是1，那麼它應該加入表TimeRegistration否則它應該加入DrivingRegistration。

解決方案：

在我的SELECT語句中我進行了以下連接：

INNER JOIN RegistrationTypes AS RegT ON R.RegistrationTypeId = RegT.Id 
LEFT OUTER JOIN TimeRegistration AS TReg ON TReg.RegistreringsId = R.Id AND RegT.Type = 1 
LEFT OUTER JOIN DrivingRegistration AS DReg ON DReg.RegistreringsId = R.Id AND RegT.Type <>1 

然後我編輯我where-clause輸出正確，取決於RegType，就像這樣：

WHERE (CASE RegT.Type WHEN 1 THEN TReg.RegistreringsId ELSE DReg.RegistreringsId END = R.Id) 

Answer 1

嘗試把兩個表中查詢中使用LEFT JOIN的現在

LEFT JOIN TimeRegistration TR ON r.rid = TR.Id AND RegT.type =1 
LEFT JOIN DrivingRegistration DR ON r.rid = DR.Id AND RegT.type <>1 

，在您選擇條款，使用

CASE RegType.Type WHEN 1 THEN TR.SomeField ELSE DR.someField END as SomeField 

另一種選擇是使用動態SQL

Answer 2

您可能需要執行兩個左連接，一個連接到TimeRegistration和一個到DrivingRegistration，並返回從相應的欄目連接表是這樣的：

LEFT JOIN TimeRegistration ON TimeRegistration.RegistreringsId = R.Id 
LEFT JOIN DrivingRegistration ON DrivingRegistration.RegistreringsId = R.Id 

，你的SELECT語句將是這樣的：

SELECT CASE WHEN RegT.Type = 1 THEN TimeRegistration.Foo ELSE DrivingRegistration.Bar END 

我喜歡你」重新嘗試去做，但我不認爲SQL很聰明。

Answer 3

SELECT 
    R.foo, tr.bar 
FROM 
    SomeTable AS R 
    INNER JOIN RegistrationTypes AS RegT ON R.RegistrationTypeId = RegT.Id 
              AND RegT1.Type = 1 
    INNER JOIN TimeRegistration AS tr ON /* whatever */ 

UNION 

SELECT 
    R.foo, dr.bar 
FROM 
    SomeTable AS R 
    INNER JOIN RegistrationTypes AS RegT ON R.RegistrationTypeId = RegT.Id 
              AND RegT1.Type = 2 
    INNER JOIN DrivingRegistration AS dr ON /* whatever */