title: ADF Mapping Data Flow byNames expression exception
body: <p>Azure Data Factory(ADF) Mapping Data Flow byNames expression is throwing an exception in the derived column block. Actually I need to access multiple columns values in a single derived column.</p>
  <pre><code>toString(byNames(['parent', 'child']))
  Exception: DF-TX-115 - Variable results are allowed in assignments - EXE-0001,[390 436 536 677],
             Dataflow cannot be analyzed as a graph,
  </code></pre>
  <p><a href="https://i.stack.imgur.com/Fr1kY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Fr1kY.png" alt="enter image description here" /></a></p>
  <p><a href="https://docs.microsoft.com/en-us/azure/data-factory/data-flow-expression-functions#bynames" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/azure/data-factory/data-flow-expression-functions#bynames</a></p>