title: how Jooq distinct or countDistinct more than one column
body: <p>I found that the distinct function in the Factory class only accepts one parameter</p>
  
  <pre class="lang-java prettyprint-override"><code>public static AggregateFunction&lt;Integer&gt; countDistinct(Field&lt;?&gt; field);
  </code></pre>
  
  <p>and there is no function like <code>distinct(Field... fields)</code></p>
  
  <p>I want to to express a SQL statement like this:</p>
  
  <pre class="lang-sql prettyprint-override"><code>select c1, count(distinct c2,c3) from t1 group by c1;
  </code></pre>
  
  <p>PS: there is a non-dsl api: <code>SelectQuery.setDistinct(true)</code>, but it will "distinct" all columns, which is not what I want.</p>