A few different SQL approaches for calculating a percent of total.

percent_of_totals_snowflake.sql

-- A percent of total calculation using a joined, aggregated subquery
select state
     , county
     , 100 * population / state_population as pct_state_population
from census
join (select state, sum(population) state_population from census group by 1) using (state)
;

-- A percent of total calculation using a windowed aggregate
select state
     , county
     , sum(population) over (partition by state) state_population
     , 100 * population / state_population as pct_state_population
from census
;

-- A percent of total calculation using a correlation in the SELECT list
select state
     , county
     , (select sum(population) from census i where o.state = i.state) state_population
     , 100 * population / state_population as pct_state_population
from census o
;