title: PHP & MySQL - Checking if database entry does NOT exist?
body: <p>I'm collecting the birthdates of users in my system, doing so by linking the the user's unique ID to their birthday entry in another table.</p>
  
  <p>To prevent users from accidentally/purposefully entering two birthdate entries for their accounts, I'd like to remove to the entry form for birthdays <strong>IF</strong> the user has already entered a birthday prior.</p>
  
  <p>For instance:</p>
  
  <pre><code>$value = mysqli_query("SELECT bd_user_id FROM user_birthdate WHERE
  bd_user_id="$user_id";");
  </code></pre>
  
  <p>From that data, how will I be able to return some form value to determine if whether the user's ID has already been index in <strong>user_birthdate</strong> or not? (Where <strong>$user_id</strong> = The current user's ID)</p>
  
  <p>Or perhaps I'm taking the wrong approach here? The logic behind it is what's been getting me.</p>
  
  <p>How can I check if whether a value is <strong>NOT</strong> indexed in a database table?</p>