Error converting data type varchar to bigint.

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C#

SELECT distinct r.rate as Rate,
f.bfid as Batchid,b.bthno,
CASE  WHEN r.rate = 1 THEN 100 
      WHEN r.rate = 2 THEN 75 
      WHEN r.rate = 3 THEN 50 
      WHEN r.rate = 4 THEN 25 
      ELSE 0 
END AS Translated_Rate 
FROM bthfac AS f,batch AS b, faculty AS fa,facfeedback AS r 
WHERE month(b.examdate)= 9 and year(b.examdate)= 2011 and b.bthid=f.bthid 
      and r.bfid = f.bfid  and fa.facid=f.facid and b.class = 'PST' and fa.facname = 'MADHAVAN';

when i run the above query shows error as follows;

Error converting data type varchar to bigint.

The above error showing because as follows,

FAculty table design structure as follows

Facid bigint (data type)


Bthfac table design structure as follows

Facid varchar (data type)


thats why error shows, for that how can i resolve and get the output.

how can i do? please help me.

Regards,
Narasiman P.