Mismatched Input 'From' Expecting Sql

In the 4th line of you code, you just need to add a comma after a.decision_id, since row_number() over is a separate column/function.

P.S.: Try yo use indentation in nested select statements so you and your peers can understand the code easily. Cheers!

I think your issue is in the inner query. You have a space between a. and decision_id and you are missing a comma between decision_id and row_number().

Is this what you want?

 SELECT
    a.ACCOUNT_IDENTIFIER,
    a.LAN_CD,
    a.BEST_CARD_NUMBER,
    decision_id, 
    CASE WHEN a.BEST_CARD_NUMBER = 1 THEN 'Y' ELSE 'N' END AS best_card_excl_flag 
 FROM (
    SELECT
        a.ACCOUNT_IDENTIFIER,
        a.LAN_CD,
        a.decision_id,
        row_number() OVER (partition BY CUST_GRP_MBRP_ID ORDER BY COALESCE(BEST_CARD_RANK,999)) AS BEST_CARD_NUMBER 
    FROM Accounts_Inclusions_Exclusions_Flagged A
 ) A; 

I want to say this is just a syntax error. I think it is occurring at the end of the original query at the last FROM statement. Of course, I could be wrong.

Try the following:

'SELECT a.ACCOUNT_IDENTIFIER, a.LAN_CD, a.BEST_CARD_NUMBER, decision_id,
    CASE    
        WHEN a.BEST_CARD_NUMBER = 1 THEN 'Y'
        ELSE 'N'
    END AS best_card_excl_flag
FROM (SELECT ROW_NUMBER() OVER(PARTITION BY CUST_GRP_MBRP
                                ORDER BY COALESCE(BEST_CARD_RANK, 999)) AS 
BEST_CARD_NUMBER,
            a.ACCOUNT_IDENTIFIER, a.LAN_CD, a.decision_id
    FROM Accounts_Inclusions_Exclusions_Flagged) AS a;'

I hope this helps some.

FROM should not be in the last sentence.