title: How the query works?
body: <p>I'm not understanding how a query in an example works. The query is like this:</p>
  
  <pre><code>SELECT COUNT(1) FROM (
      SELECT 1
      FROM Table1
      WHERE... 
      GROUP BY Table1.column2, Table1. Column3
  )
  </code></pre>
  
  <p>Do you know how this query works in practically, particularly the select count (1) and select 1? </p>