title: BigQuery - cannot UNION String and NULL
body: <p>I have two tables that I am trying to <code>UNION</code>. Column <code>id</code> in <code>table1</code> is of type <code>STRING</code>. I do not have that column in <code>table2</code> so I am writing <code>SELECT NULL AS id FROM table2</code> but BigQuery thinks that column <code>id</code> in <code>table2</code> is of type <code>INT64</code>.</p>
  
  <p>I get this error -
  <code>Column 2 in UNION ALL has incompatible types: STRING, INT64</code>.</p>
  
  <p>Query works if I write <code>SELECT "" AS id FROM table2</code> instead.</p>