How to find maximum avg

Columns resulting from aggregate functions (e.g. avg) usually get arbitrary names. Just use an alias for it, and select on that:

select max(avg_salary)
from (select worker_id, avg(salary) AS avg_salary
      from workers
      group by worker_id) As maxSalary;

select worker_id, avgsal 
from 
(
  select worker_id, avg(salary) as avgsal 
  from workers 
  group by worker_id
) 
where avgsal=(select  max(avgsal) 
              from (select worker_id, avg(salary) as avgsal 
                    from workers group by worker_id))

This will display the highest average along with worker id

As explained here you can use

SELECT worker_id, AVG(salary)
FROM workers
GROUP BY worker_id
HAVING AVG(salary) = (SELECT MAX(AVG(salary)) FROM workers GROUP BY worker_id)