title: Convert value ' ' to null in SQL or BigQuery
body: <p>I have the following extraction query</p>
  
  <pre><code>select A.documentid.Docid, A.documentId.Appid, A.timestamp, A.EventStatus, D.Sequence, D.EventAppName, E.Value as Federation
  
  from `dbo.events` A left join `dam.eventsroot` B on A.documentid.docid = B.docid left join `dbo.documentroot` C on B.rootdocid=C.rootdocid 
  inner join `dbo.reference_status` D
  
  on A.DocumentID.AppID=D.EventAppID and A.EventStatus = D.EventStatus left join unnest(C.metadata) E on E.Key='Federation'
  
  where A.timestamp &gt; TIMESTAMP_TRUNC(CURRENT_TIMESTAMP(),DAY)
  </code></pre>
  
  <p>The result is that in column FEDERATION i get different values and values of '' (nothing) and i want to change them to Null.</p>
  
  <p>How can i do this?</p>
  
  <p>Thank you very much</p>