select grade2.name,grade2.chinese,grade2.math,grade2.english,
(case chinese when 'NA' then 0 else convert(integer,chinese) end +
case math when 'NA' then 0 else convert(integer,math) end +
case english when 'NA' then 0 else convert(integer,english) end
) /
(case chinese when 'NA' then 0 else 1 end +
case math when 'NA' then 0 else 1 end +
case english when 'NA' then 0 else 1 end
)
from grade2