我试过以下查询,但它给了我错误.

SELECT RQ.int_REPS_QUES_DIFF_LEVEL,SUM(1) AS NoOFDificultyQuestion FROM   
 REPS_TEST_QUES_ASSIGNED RQA   
 INNER JOIN REPS_QUESTION RQ ON RQA.int_REPS_TEST_QUES_ASSG_QUESID=RQ.PK_REPS_QUES_ID  
 WHERE int_REPS_TEST_ID IN(  
 SELECT FK_TEST_ID FROM STUDENT_EXAM SE WHERE FK_USER_ID=56 AND SE.FK_REPS_BATCH_ID=466 
    and CASE 
WHEN @FromDate!=NULL AND @ToDate!=NULL     
     THEN dat_STUD_EXAM_FINALEND >= @FromDate AND dat_STUD_EXAM_FINALEND <= @ToDate 
WHEN @FromDate!=NULL AND @ToDate=NULL  
     THEN  dat_STUD_EXAM_FINALEND >= @FromDate  
WHEN @FromDate=NULL AND @ToDate!=NULL  
     THEN  dat_STUD_EXAM_FINALEND <= @ToDate
END
 )   
 strong textGROUP BY RQ.int_REPS_QUES_DIFF_LEVEL

我在收到错误

THEN dat_STUD_EXAM_FINALEND >= @FromDate AND dat_STUD_EXAM_FINALEND <= @ToDate

这行我告诉我哪里出错了