Just a little improvement ...

SELECT S.Clm_Id, S.Svc_Dt, P.Active_Ind ;
FROM Mc_MdSvc S Inner Join Mc_MdClm C on C.Clm_Id = S.Clm_Id
LEFT JOIN Mc_PlnHs P ON P.Mem_Num = C.Mem_Num ;
AND P.Pln_Id = C.Pln_Id ;
AND P.Ins_Id_Num = C.Ins_Id_Num ;
WHERE C.Adjt_Stat = "Z" ;
AND P.Active_Ind = .T. ;
AND Between(S.Svc_Dt, P.Eff_From, P.Eff_Thru) ;
HAVING ISNULL(P.Active_Ind)

Nuno Brito

Note: I didn´t tested, but i think it work.