title: How to fire an exception when a duplicated value is tried to be inserted (without constraints usage)?
body: <p>In an interview somebody asked me how to create a table that throws an exception when a duplicated value is tried to be inserted in a column, but no constraints should be used. This is in Oracle.</p>
  
  <p>It might be a dumb question, but how do you achieve this?</p>
  
  <p>Thanks.</p>