title: Not able to add unique to products table through Laravel migration
body: <p><a href="https://i.stack.imgur.com/KrP2C.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KrP2C.png" alt="enter image description here"></a></p>
  
  <p>I am trying to add <code>unique</code> to <code>slug</code> in products table and I have the following migration. However when I run <code>php artisan migrate</code> I get an error.</p>
  
  <pre><code>use Illuminate\Support\Facades\Schema;
  use Illuminate\Database\Schema\Blueprint;
  use Illuminate\Database\Migrations\Migration;
  
  class AddUniqueToProductsTable extends Migration
  {
      /**
       * Run the migrations.
       *
       * @return void
       */
      public function up()
      {
          Schema::table('products', function (Blueprint $table) {
             $table-&gt;unique('slug'); 
          });
      }
  
      /**
       * Reverse the migrations.
       *
       * @return void
       */
      public function down()
      {
          Schema::table('products', function (Blueprint $table) {
              $table-&gt;dropUnique('products_slug_unique');
          });
      }
  }
  </code></pre>
  
  <p>Error</p>
  
  <blockquote>
    <p>➜ php artisan migrate</p>
    
    <p>[Illuminate\Database\QueryException]   SQLSTATE[23000]: Integrity
    constraint violation: 1062 Duplicate entry '' for key
    'products_slug_unique' (SQL: alter table <code>products</code> add    unique
    <code>products_slug_unique</code>(<code>slug</code>))</p>
    
    <p>[Doctrine\DBAL\Driver\PDOException]   SQLSTATE[23000]: Integrity
    constraint violation: 1062 Duplicate entry '' for key
    'products_slug_unique'</p>
    
    <p>[PDOException]   SQLSTATE[23000]: Integrity constraint violation:
    1062 Duplicate entry '' for key 'products_slug_unique'</p>
  </blockquote>
  
  <p>What am I doing wrong here?</p>