有条件的内部联接

Question

我希望能够根据表达式的结果内联两个表。

我一直在试图至今：

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

REGT是加入我做之前这个连接：

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

此SQL脚本不起作用。

所以总而言之，如果Type是1，那么它应该加入表TimeRegistration否则它应该加入DrivingRegistration。

解决方案：

在我的SELECT语句中我进行了以下连接：

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输出正确，取决于RegType，就像这样：

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

Answer 1

尝试把两个表中查询中使用LEFT JOIN的现在

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 

，在您选择条款，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，一个连接到TimeRegistration和一个到DrivingRegistration，并返回从相应的栏目连接表是这样的：

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

，你的SELECT语句将是这样的：

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

我喜欢你”重新尝试去做，但我不认为SQL很聪明。

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 */