想笑的请试试这个

UPDATE Voters
SET PrecinctCode = FaultCode
WHERE PrecinctCode = '999' and FaultCode <> '999'

或者试试

UPDATE Voters
SET PrecinctCode = RTRIM (FaultCode)
WHERE PrecinctCode = '999' 

和

select *, len(FaultCode)
from Voters
WHERE PrecinctCode = '999' 
  and PrecinctCode <> FaultCode