title: Single-row subquery returns more than one row error for COUNT() function
body: <p>I am trying to understand why this would throw an error-</p>
  
  <pre><code>select count((select 1 br from dual
                 union
                select 2 br from dual)) 
    from dual; 
  
  
  ORA-01427: single-row subquery returns more than one row
  </code></pre>
  
  <p>But This won't-</p>
  
  <pre><code>select count(br)
  from (select 1 br from dual
         union
        select 2 br from dual);  
  
  BR
  ----------
  2  
  </code></pre>