Get list of characters in column
select string_agg(c,'')
from (
  select distinct regexp_split_to_table(lower(name),'') as c
  from data
) t


The inner select generates one row for each character, and the outer then aggregates that to a long string. 

If you want the characters sorted, you can use an order by for the aggregate string_agg(c,'' order by c)