如何在then子句中的select查询中选择和比较值

Question

我正在从From t2 OI Join t3 OPL ON OI.c1 = OPL.P1 join t4 sp on sp.c2 = OI.p2 where sp.ID ='IQ')获取第1组产品，ID 它适用于Group1和Product，但带有ID我面临问题 我想将ID与R.ID进行比较，然后需要检索该值 我该怎么办？

 (SELECT Top 1 (CASE WHEN Group1='p'
             THEN (Case when Product='10'
                    THEN(Select top 1 Email from t1 as R join  t2 as E
                            on R.C1=E.E1 where R.ID=ID
                        )

                ELSE (Select top 1 Email from t1 as R join  t2 as E
                        on R.B1=E.E1 where R.ID=ID      
                    ) 
                END) 
    ELSE
    (Select top 1 Email from t1 as R join  t2 as E
                        on R.A1=E.E1 where R.ID=ID 
    )

    END) AS Email
From t2 OI 
                      Join t3 OPL ON OI.c1 = OPL.P1
                      join t4 sp on sp.c2 = OI.p2
                      where sp.ID ='IQ')