如果值存在连接表，则为条件连接

Question

我有一个名为level_2_tags的表，如果用户选择了二级标记我希望它通过内部联接加入另一个表，否则我希望它加入左连接

IF RLT ID EXISTS BELOW METHOD 
SELECT  RMC.* INTO #TTEMP   
FROM    #TRestrictedMainCatagory RMC
    JOIN #TRestrictedAssetType RAT
        ON RMC.ID = RAT.ID  
    JOIN #TRestrictedLevelTwo RLT
        ON RLT.ID=RAT.ID

ELSE 
     SELECT RMC.* INTO #TTEMP   
FROM    #TRestrictedMainCatagory RMC
    JOIN #TRestrictedAssetType RAT
        ON RMC.ID = RAT.ID  
    LEFT OUTER JOIN #TRestrictedLevelTwo RLT
        ON RLT.ID=RAT.ID

请帮我解决此事