title: not single-group grouping function while using regr_slope
body: <p>HI I try to run : </p>
  
  <pre><code> select
    year,
    regr_slope(sum(sale_count),year) as slope,
  from products
  group by year
  </code></pre>
  
  <p>It throws "<strong><em>00937. 00000 -  "not a single-group group function"</em></strong>" .When i delete year from select clause problem disapears. Shouldn't I be able to select column with which I'm grouping?</p>
  
  <p>Oracle 11.2 sqldeveloper</p>
  
  <p>ty for help !</p>