title: Calculates the difference between employee salary and the average salary of job group
body: <p>Given a table like:</p>
  
  <pre><code>Employee (emp_no, emp_fname, emp_lname, emp_salary, job_title) 
  </code></pre>
  
  <p>How to write a query that for each employee calculates the difference
  between his/her salary to the average salary of his/her job group?</p>
  
  <p>To get the avg. Salary for each group I use:</p>
  
  <pre><code>Select job_title, avg(emp_salary) as avg_salary 
  from employee group by job_title;
  </code></pre>
  
  <p>But I'm struggling to find the difference between salary and avg. salary for each job title.</p>