title: PL SQL String comparison
body: <p>I'm writing a procedure in PL/SQL, where I need to check whether a given value does not equal <em>x</em> or <em>y</em>. I use the following code to check:</p>
  
  <pre><code>IF p_campaignPaymentType != 'per month' || 'per creation' THEN
      RAISE ex_invalidPaymentType;
  END IF;
  </code></pre>
  
  <p>Now, even if the string given in p_campaignPaymentType is 'per month' or 'per creation', it raises the exception ex_invalidPaymentType.</p>
  
  <p>What is the problem with the IF-sentence, and/or what could I do to as an alternative for checking?</p>