title: about "data merge" in SAS
body: <p>I am studying data merge in SAS, and find the following example</p>
  
  <pre><code>data newdata;
  merge yourdata (in=a) otherdata (in=b);
  by permno date;   
  </code></pre>
  
  <p>I do not know what do "(in=a)" and "(in=b)" mean? Thanks.</p>