title: Checkbox multiple selection filter
body: <p>I want to filter the data into my program depending what checkbox is selected or not.</p>
  
  <pre><code>parameters: p_z1 as checkbox DEFAULT 'X' ,
              p_z2 as checkbox.
  selection-screen end of block b4.
  </code></pre>
  
  <p>So if one of these two is selected or if both are selected how can I filter my data?</p>
  
  <pre><code> select single * from mara where matnr = pv_matnr 
  and "if checkbox one is selected" where matkl = t023-matkl.
  "if checkbox two is selected" where matkl = v023-matkl.
  </code></pre>