title: Do I need an else clause in a case expression?
body: <p>Does one need to include an <code>ELSE</code> clause in a <code>CASE</code> expression? </p>
  
  <p>For example, if I wanted to pull the names of animals that are cats and nothing ELSE, could I use this <code>SELECT</code> statement: </p>
  
  <pre><code>SELECT DISTINCT(CASE WHEN animal_type = 'cat' THEN animal_name END) AS cat_names
  </code></pre>
  
  <p>I know I could just put <code>animal_type = 'cat'</code> in my <code>WHERE</code> clause and then </p>
  
  <pre><code>SELECT DISTINCT cat_names, 
  </code></pre>
  
  <p>but I'd like to know the answer. </p>