title: Subquery returned more than 1 value causing query to fail
body: <p>I'm stuck with this query. i want to show a field called "Executive" that must say "sin asignar" if there's no match when joining on the table 'prospectousuario', and if it exist it must say the first and the last name of the executive. But it returns the following error:</p>
  
  <blockquote>
    <p>Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, &lt;, &lt;= , >, >= or when the subquery is used as an exp</p>
  </blockquote>
  
  <p>The query is:</p>
  
  <pre><code>select p.IDCANCAP, 
  p.CEDULA, 
  p.NOMBRES, 
  p.APELLIDOS, 
  p.CELULAR,
  p.CASA, 
  p.CORREO, 
  p.ESTABLECIMIENTO, 
  c.DESCRIPCION,  
  (select case when
  p.CEDULA = pu.IDPROSPECTO and pu.IDUSUARIO = u.CEDULA 
  then u.NOMBRES+' '+u.APELLIDOS else 'Sin asignar' 
  end from usuario as u, PROSPECTOUSUARIO as pu) as Ejecutivo,
  p.FECHA_CREAC 
  
  from PROSPECTO p, CANALCAPTACION c 
  where p.IDCANCAP = c.IDCANAL
  </code></pre>