Using width_bucket function to bin in postgres

postgres_binning.sql

with example as (
    select 399 as num, width_bucket(399, 400, 990, 59) as buk
    union all
    select 400 as num, width_bucket(400, 400, 990, 59) as buk
    union all
    select 401 as num, width_bucket(401, 400, 990, 59) as buk
    union all
    select 409 as num, width_bucket(409, 400, 990, 59) as buk
    union all
    select 410 as num, width_bucket(410, 400, 990, 59) as buk
    union all
    select 411 as num, width_bucket(411, 400, 990, 59) as buk
    union all
    select 989 as num, width_bucket(989, 400, 990, 59) as buk
    union all
    select 990 as num, width_bucket(990, 400, 990, 59) as buk
    union all
    select 991 as num, width_bucket(991, 400, 990, 59) as buk
    union all
    select 999 as num, width_bucket(999, 400, 990, 59) as buk
)
select 
    num, 
    case when buk = 0 then '[0-400)'
    when buk >= 60 then '[990-999]'
    else '[' || (400 + (10 * buk-1)) || '-' || (400 + (10 * buk-1)) || ')'
    end
from
    example;