问Laravel-4迁移中的访问冲突错误

EN

class CreateUsersTable extends Migration {
    public function up()
    {
        Schema::create('users', function(Blueprint $table) {
            $table->increments('id');
            $table->string('name', 255);
            $table->string('username', 64)->unique();
            $table->string('email', 255)->unique();
            $table->string('password',64);
            $table->timestamps();
        });
    }

    public function down()
    {
        Schema::drop('users');
    }

}

class CreatePredictionsTable extends Migration {
    public function up()
    {
        Schema::create('predictions', function(Blueprint $table) {
            $table->increments('id');
        $table->integer('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users');
        $table->integer('fixture_id')->unsigned();
        $table->foreign('fixture_id')->references('id')->on('fixtures');
        $table->integer('home_team_score_prediction');
        $table->integer('away_team_score_prediction');
        $table->timestamps();
        });
    }
    public function down()
    {
        Schema::drop('predictions');
    }
}

class CreateFixturesTable extends Migration {
    public function up()
    {
        Schema::create('fixtures', function(Blueprint $table) {
            $table->increments('id');
            $table->integer('home_team_id');
            $table->integer('away_team_id');
            $table->dateTime('date');
            $table->string('venue');
            $table->timestamps();
        });
    }
    public function down()
    {
        Schema::drop('fixtures');
    }
}

 php artisan migrate

php artisan migrate:refresh

SQLSTATE[42S01]: Base table or view already exists: 1050 Table 'predictions' already exists (SQL: create table `predictions` (`id` int unsigned not null auto_increment primary key, `user_id` int unsigned not null, `fixture_id` int
  unsigned not null, `home_team_score_prediction` int not null, `away_team_score_prediction` int not null, `created_ at` timestamp default 0 not null, `updated_at` timestamp default 0 not null) default character set utf8 collate utf8_unicode_ci)

php artisan migrate:rollback

 [Illuminate\Database\QueryException]
  SQLSTATE[23000]: Integrity constraint violation: 1217 Cannot delete or update a parent row: a foreign key constrain t fails (SQL: drop table `users`)