title: SQL query include in CONCAT only if cell is not empty
body: <p>I am building using <code>CONCAT</code> a long string, one cell from DB (<code>design_name</code>) is in some rows empty (not <code>NULL</code>, just empty). Every single value is separated by <code>_</code>
  I would like to ignore the value when this cell is empty, because in this case I have double <code>__</code> sign in final string. Itried using <code>CASE</code> but I get errors.</p>
  
  <pre><code>SELECT CONCAT(year,t,id, ' - ', country, '_', year, '_',value, '_', order, 
  CASE WHEN design_name != '' THEN '_', design_name, END '_', customer, '_', order) 
  AS customer_projects
  FROM projects WHERE customer = 'SAMPLE_CUSTOMER' 
  ORDER BY id DESC
  </code></pre>