Collection.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

/**
 * Tax report collection
 *
 * @author      Magento Core Team <core@magentocommerce.com>
 */
namespace Magento\Tax\Model\ResourceModel\Report;

class Collection extends \Magento\Sales\Model\ResourceModel\Report\Collection\AbstractCollection
{
    /**
     * @var \Zend_Db_Expr
     */
    protected $_periodFormat;

    /**
     * Aggregated Data Table
     *
     * @var string
     */
    protected $_aggregationTable = 'tax_order_aggregated_created';

    /**
     * @var array
     */
    protected $_selectedColumns = [];

    /**
     * @param \Magento\Framework\Data\Collection\EntityFactory $entityFactory
     * @param \Psr\Log\LoggerInterface $logger
     * @param \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy
     * @param \Magento\Framework\Event\ManagerInterface $eventManager
     * @param \Magento\Sales\Model\ResourceModel\Report $resource
     * @param mixed $connection
     */
    public function __construct(
        \Magento\Framework\Data\Collection\EntityFactory $entityFactory,
        \Psr\Log\LoggerInterface $logger,
        \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
        \Magento\Framework\Event\ManagerInterface $eventManager,
        \Magento\Sales\Model\ResourceModel\Report $resource,
        \Magento\Framework\DB\Adapter\AdapterInterface $connection = null
    ) {
        $resource->init($this->_aggregationTable);
        parent::__construct($entityFactory, $logger, $fetchStrategy, $eventManager, $resource, $connection);
    }

    /**
     * @return array
     */
    protected function _getSelectedColumns()
    {
        if ('month' == $this->_period) {
            $this->_periodFormat = $this->getConnection()->getDateFormatSql('period', '%Y-%m');
        } elseif ('year' == $this->_period) {
            $this->_periodFormat = $this->getConnection()->getDateFormatSql('period', '%Y');
        } else {
            $this->_periodFormat = $this->getConnection()->getDateFormatSql('period', '%Y-%m-%d');
        }

        if (!$this->isTotals() && !$this->isSubTotals()) {
            $this->_selectedColumns = [
                'period' => $this->_periodFormat,
                'code' => 'code',
                'percent' => 'percent',
                'orders_count' => 'SUM(orders_count)',
                'tax_base_amount_sum' => 'SUM(tax_base_amount_sum)',
            ];
        }

        if ($this->isTotals()) {
            $this->_selectedColumns = $this->getAggregatedColumns();
        }

        if ($this->isSubTotals()) {
            $this->_selectedColumns = $this->getAggregatedColumns() + ['period' => $this->_periodFormat];
        }

        return $this->_selectedColumns;
    }

    /**
     * Apply custom columns before load
     *
     * @return $this
     */
    protected function _beforeLoad()
    {
        $this->getSelect()->from($this->getResource()->getMainTable(), $this->_getSelectedColumns());
        if (!$this->isTotals() && !$this->isSubTotals()) {
            $this->getSelect()->group([$this->_periodFormat, 'code', 'percent']);
        }

        if ($this->isSubTotals()) {
            $this->getSelect()->group([$this->_periodFormat]);
        }
        return parent::_beforeLoad();
    }
}