如何在向表中添加新列的迁移中为现有行插入不同于缺省值的值

<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Schema;

class AddAgeColumnToUsersTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        // 添加新列
        Schema::table('users', function (Blueprint $table) {
            $table->integer('age')->default(18);
        });

        // 更新已存在的行
        DB::statement("
            UPDATE users
            SET age = 18
            WHERE age IS NULL
        ");
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        // 删除新列
        Schema::table('users', function (Blueprint $table) {
            $table->dropColumn('age');
        });
    }
}