title: Add mysql unique constraints to apply to entries within a certain date
body: <p>Am trying to add constraints to a mysql table, however i want this constraints to apply only to entries after a certain date. i.e the constraints is for all entries after <code>'2020-01-01 00:00:00'</code> and not before.</p>
  <p>This the query am using, how do i add the time check if its possible. Thank you</p>
  <pre><code>ALTER TABLE table_name ADD UNIQUE unique_index(col1, col2, col3) ;
  </code></pre>