with culc as (
    select count(*) as num, avg(point) as average
    from test_results
    where test_id = '100'
)
, stdev as (
    select sqrt(avg(power(point - average, 2))) as value
    from test_results
    cross join culc
)
select user_id as USER, point as PT, 50 + case value when 0 then 0 else round((point - average) * 10 / value, 1) end as DEV_VAL
from test_results, culc, stdev