title: Combine Rows in BigQuery Ignoring Nulls
body: <p>I have a Google BigQuery table that looks like this:</p>
  
  <pre><code>║ id ║   col_1    ║  col_2  ║ updated ║
  
  ║  1 ║ first_data ║ null    ║ 4/22    ║
  
  ║  1 ║ null       ║ old     ║ 4/23    ║
  
  ║  1 ║ null       ║ correct ║ 4/24    ║
  </code></pre>
  
  <p>I would like to construct a query that combines these rows and "overwrites" null columns if there is a row with the same id with the column not null. Essentially the result should look like:</p>
  
  <pre><code>║  1 ║ first_data ║ correct ║ 4/24    ║
  </code></pre>
  
  <p>If possible I would also like the result to represent a history:</p>
  
  <pre><code>║  1 ║ first_data ║ old     ║ 4/23    ║
  
  ║  1 ║ first_data ║ correct ║ 4/24    ║
  </code></pre>
  
  <p>But that is secondary and not necessary.</p>