title: case when statement not working in bigquery
body: <p>I'm getting the following error message when I do a case when statement:</p>
  
  <pre><code>No matching signature for operator CASE for argument types: BOOL, FLOAT64, 
  STRING at [10:1] 
  </code></pre>
  
  <p>the statement is:</p>
  
  <pre><code>case when a.datetime between '2015-11-01' and '2016-10-31' then 
  a.i_sec_out_to_auth else 'NA' end as PY_resp,
  case when a.datetime between '2016-11-01' and '2017-10-31' then 
  a.i_sec_out_to_auth  else 'NA' end as CY_resp
  </code></pre>
  
  <p>Am I missing something. is google bigquery not handling case whens fully yet?</p>
  
  <p>Keep in mind a.i_sec_out_to_auth is a column of type float in my dataset. </p>