有条件的内部加入

Question

我希望能够根据表达式的结果内部连接两个表.

到目前为止我一直在努力:

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

RegT是我在此加入之前创建的联接:

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

此SQL脚本不起作用.

总而言之,如果Type是1,那么它应该加入表中,TimeRegistration否则它应该加入DrivingRegistration.

解:

在我的select语句中,我执行了以下连接:

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

然后我编辑我where-clause的输出正确,取决于RegType,像这样:

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

Answer 1

尝试使用LEFT JOIN将两个表放入查询中

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 

现在,在你的select子句中,使用

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

另一种选择是使用动态SQL

Answer 2

您可能需要执行两个左连接,一个进入TimeRegistration和一个进入DrivingRegistration,并从相应的连接表返回您想要的字段,如下所示:

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

你选择语句会是这样的:

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

我喜欢你想做的事,但我不认为SQL是那么聪明.