CASE statement and sub-query aggregate

case_agg.sql

SELECT salary_group, count(*) FROM
    (SELECT
        employee_id,
        last_name,
        CASE
              WHEN salary <= 3000 THEN 'poor'
              WHEN salary >3000 AND salary <= 11000 THEN 'okay'
              WHEN salary > 11000 THEN 'rich'
              ELSE 'Theres a problem'
        END AS salary_group
      FROM hr.employees) x
GROUP BY salary_group;