SQL CASE STATEMENT IMPLEMENTATION

REquirment
•       Add indicator to the end of the files, (position 1051 in both extracts, either have value of Y or N).  
o       DUAL COVERAGE  (Y/N)
•       Logic to determine indicator values from Member Eligibility Table:
o       For ‘Y’ indicator:
o       When dbo. CMC_MEPE_PRCS_ELIG.CSCS_ID = ‘MD11’ (Mean Dual covered member)
o       For ‘N’ indicator:
        When dbo. CMC_MEPE_PRCS_ELIG.CSCS_ID <> ‘MD11’ 
•       Groups included:
o       100100 (Medicare)
o       100598 (Medicaid-OHP)
•       Previous syntax
•       select top 20000
•                       EXT_DATE,SUPPLIER_NAME,BUSINESS_UNIT,PROG_NO,PG_DESC,DIVISION_NO,RISKPOP_MEDICARE_EVENT_CODE,
•                       RISKPOP_MEDICAID_AID_CATEGORY,MEMBER_NO,MEDICARE_NO,MEDICAID_NO,LASTNAME,MIDINITIAL,FIRSTNAME,YMDBIRTH,
•                       a.SEX,CLASS,CL_DESC,S_LASTNAME,S_MIDINITIAL,S_FIRSTNAME,S_DOB,S_SEX,BENFIT_PKG,BEN_PKG_DESC,CLASS_ID,PLAN_ID,
•                       YMDEFF_BP,YMDEND,REASON,PRIME_SECOND,PHP_DUAL,S_ADDRESS1,S_ADDRESS2,S_ADDRESS3,S_CITY,S_STATE,S_ZIP,S_COUNTRY,
•                       S_PHONE,M_ADDRESS1,M_ADDRESS2,M_ADDRESS3,M_CITY,M_STATE,M_ZIP,M_COUNTRY,M_PHONE,DIV_NAME,PROV_NO,IRS_NO,
•                       P_LASTNAME,P_FIRSTNAME,P_MIDINITIAL,YMDEFF_PCP,ALT_KEY,ALT_CARRIER_NO,ALT_CARRIER_FUL,ALT_YMDEFF,ALT_YMDEND,
•                       BENFIT_PKG_EFF,BENFIT_PKG_TERM,a.MEME_CK,
•       b.HIOS_NUMBER,
•       null as PREMIUM_PAID_DATE,
•       null as PREMIUM_PAID_END_DATE,
•       b.APTC,
•       null as 'GracePeriodIndicatorStartDate',
•       null as'GracePeriodIndicatorEndDate', 
•       null as'ExchangePlanType' 
•       from Extract_602_603_PBH_Extract_Main a
•       left join Extract_602_603_PBH_Extract_HRI b
•       on LEFT(a.MEMBER_NO,7) = b.SBSB_ID

Syntax I am trying to implement(taking forever to execute) Please help correct

select distinct
                EXT_DATE,SUPPLIER_NAME,BUSINESS_UNIT,PROG_NO,PG_DESC,DIVISION_NO,RISKPOP_MEDICARE_EVENT_CODE,
                RISKPOP_MEDICAID_AID_CATEGORY,MEMBER_NO,MEDICARE_NO,MEDICAID_NO,LASTNAME,MIDINITIAL,FIRSTNAME,YMDBIRTH,
                a.SEX,CLASS,CL_DESC,S_LASTNAME,S_MIDINITIAL,S_FIRSTNAME,S_DOB,S_SEX,BENFIT_PKG,BEN_PKG_DESC,CLASS_ID,PLAN_ID,
                YMDEFF_BP,YMDEND,REASON,PRIME_SECOND,PHP_DUAL,S_ADDRESS1,S_ADDRESS2,S_ADDRESS3,S_CITY,S_STATE,S_ZIP,S_COUNTRY,
                S_PHONE,M_ADDRESS1,M_ADDRESS2,M_ADDRESS3,M_CITY,M_STATE,M_ZIP,M_COUNTRY,M_PHONE,DIV_NAME,PROV_NO,IRS_NO,
                P_LASTNAME,P_FIRSTNAME,P_MIDINITIAL,YMDEFF_PCP,ALT_KEY,ALT_CARRIER_NO,ALT_CARRIER_FUL,ALT_YMDEFF,ALT_YMDEND,
                BENFIT_PKG_EFF,BENFIT_PKG_TERM,a.MEME_CK,
b.HIOS_NUMBER,
null as PREMIUM_PAID_DATE,
null as PREMIUM_PAID_END_DATE,
b.APTC,
null as 'GracePeriodIndicatorStartDate',
null as'GracePeriodIndicatorEndDate', 
null as'ExchangePlanType',
case when MEPE.CSCS_ID = 'MD11' AND grgr.GRGR_ID IN('100100','100598')
THEN 'Y'
ELSE 'N' end as DualCoverageIndicator
from Extract_602_603_PBH_Extract_Main a
left join Extract_602_603_PBH_Extract_HRI b
on LEFT(a.MEMBER_NO,7) = b.SBSB_ID
join FacetsReport.dbo.CMC_MEPE_PRCS_ELIG MEPE
ON a.CLASS_ID= MEPE.CSCS_ID
Join FacetsReport.dbo.CMC_GRGR_GROUP grgr
ON MEPE.GRGR_CK= grgr.GRGR_CK