title: WHERE NOT EXISTS in PostgreSQL gives syntax error
body: <p>When trying to use the <code>WHERE NOT EXISTS</code> clause to prevent adding a row with a duplicate value in the column <code>age</code>, I get the error <code>syntax error at or near "WHERE"</code>.</p>
  
  <p>Why did it throw a syntax error? I'm using Postgresql 9.1.</p>
  
  <p><strong>SQL</strong></p>
  
  <pre><code>INSERT INTO live.users ("website", "age") 
  values ('abc', '123')
  WHERE NOT EXISTS (SELECT age FROM live.users WHERE age = 123);
  </code></pre>
  
  <p><strong>Error</strong></p>
  
  <pre><code>ERROR:  syntax error at or near "WHERE"
  LINE 6: WHERE NOT EXISTS (SELECT age FROM live.users W...
  </code></pre>