title: Case expressions and boolean
body: <p>Why does only the first one work?</p>
  
  <ol>
  <li><p>returns 'blue':</p>
  
  <p><code>select case when 'a' = 'a' then 'blue' end from dual;</code></p></li>
  <li><p>Error: ORA-00920: invalid relational operator</p>
  
  <p><code>select case when true then 'blue' end from dual;</code></p></li>
  <li><p>ORA-00904: "TRUE": invalid identifier</p>
  
  <p><code>select case when 'a' = 'a' then true end from dual;</code></p></li>
  <li><p>ORA-00905: missing keyword</p>
  
  <p><code>select case when 'b' = 'b' then 'a' = 'a' end from dual;</code></p></li>
  </ol>