<?php

namespace vendor\module\Setup;

use Magento\Framework\Setup\InstallSchemaInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\SchemaSetupInterface;

class InstallSchema implements InstallSchemaInterface
{

/**
 * {@inheritdoc}
 * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
 */
public function install(SchemaSetupInterface $setup, ModuleContextInterface $context)
{
    $installer = $setup;

    $installer->startSetup();

    /* While module install, creates column in sales_order_grid table */

    $eavTable = $installer->getTable('sales_order_grid');

    $columns = [
        'coupon_code' => [
            'type' => \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
            'nullable' => true,
            'comment' => 'Coupon Code',
        ],
    ];

    $connection = $installer->getConnection();
    foreach ($columns as $name => $definition) {
        $connection->addColumn($eavTable, $name, $definition);
       }

    $installer->endSetup();
}
}