title: Adding a column to a Dataset<Row> using withColumn()..giving issue... missing from in operator !Project
body: <p>I am usally able to get a new dataset by adding new column to an existing dataset using <code>withColumn</code>. 
  But I am not sure why this case is giving error. </p>
  
  <pre><code>Dataset&lt;Row&gt; inputDSAAcolonly = inputDSAA.select(colNameA);
  Dataset&lt;Row&gt; inputDSBBcolonly = inputDSBB.select(colNameB);
  inputDSBBcolonly.withColumn(colNameA, inputDSAAcolonly.apply(colNameA)).show();
  </code></pre>
  
  <p>where <code>inputDSSAAcolonly</code> is </p>
  
  <pre><code>+----+
  |Exer|
  +----+
  |Some|
  |None|
  |None|
  |None|
  </code></pre>
  
  <p>and <code>inputDSSBBColonly</code> is</p>
  
  <pre><code>+-----+
  |Smoke|
  +-----+
  |Never|
  |Regul|
  |Occas|
  |Never|
  </code></pre>
  
  <p>Basically  single columns in the datasets.</p>
  
  <p>I need a DS with 2 columns side by side.
  <code>withColumn</code> has worked but this throwing error:     </p>
  
  <blockquote>
    <p>Exception in thread "main" org.apache.spark.sql.AnalysisException: 
    resolved attribute(s) Exer#7 missing from Smoke#47 in operator<br>
    !Project [Smoke#47, Exer#7 AS Exer#112];;!Project [Smoke#47, Exer#7 AS
    Exer#112]</p>
  </blockquote>