TYPO3 Core

I suggest to add a new operand that allows using the MySQL ExtractValue() function that allows retrieving a value specified by an xpath of the xml-content .
See https://dev.mysql.com/doc/refman/5.6/en/xml-functions.html#function_extractvalue

This can be particularly useful when working with flexform data, e.g. also when creating content elements with gridelements.

Example usage e.g. in SampleRepository.php:

namespace BKW\BkwTiles\Domain\Repository;

use VND\MyExtension\Domain\Db\ExtractValue;

class SampleRepository extends \TYPO3\CMS\Extbase\Persistence\Repository {
   /**
     * Returns all something elements on a page, filtered by title
     * @param string $someTitle title to filter for
     * @return array|\TYPO3\CMS\Extbase\Persistence\QueryResultInterface
     * @throws \TYPO3\CMS\Extbase\Persistence\Exception\InvalidQueryException
     */
    public function findFiltered($someTitle) {
      $query->logicalAnd([
        $query->equals('ctype', 'something'),
        $query->equalsExtractValue('//data/sheet[@index="text"]/language[1]/field[@index="title"]/value', 'pi_flexform', $someTitle)
      ]);
    }
}

So far, we implemented this by extending the Typo3DbQueryParser with the following code:

<?php

namespace VND\MyExtension\Xclass\Db;

use VND\MyExtension\Domain\Db\ExtractValueInterface;
use TYPO3\CMS\Extbase\Persistence\Generic\Qom\DynamicOperandInterface;
use TYPO3\CMS\Extbase\Persistence\Generic\Qom\JoinInterface;
use TYPO3\CMS\Extbase\Persistence\Generic\Qom\SelectorInterface;
use TYPO3\CMS\Extbase\Persistence\Generic\Qom\SourceInterface;

class Typo3DbQueryParser extends \TYPO3\CMS\Extbase\Persistence\Generic\Storage\Typo3DbQueryParser {
    /**
     * @param DynamicOperandInterface $operand
     * @param SourceInterface $source The source
     * @param array &$sql The query parts
     * @return string
     * @throws \InvalidArgumentException
     */
    protected function parseOperand(DynamicOperandInterface $operand, SourceInterface $source, array &$sql)
    {
        if ($operand instanceof ExtractValueInterface) {
            if ($source instanceof SelectorInterface) {
                // @todo Only necessary to differ from  Join
                $className = $source->getNodeTypeName();
                $tableName = $this->dataMapper->convertClassNameToTableName($className);
            } elseif ($source instanceof JoinInterface) {
                $tableName = $source->getJoinCondition()->getSelector1Name();
            }
            $constraintSQL = 'ExtractValue(' . parent::parseOperand($operand, $source, $sql) . ', '.
                $this->databaseHandle->fullQuoteStr($operand->getXpath(), isset($tableName) ? $tableName : '').')';
        } else {
            return parent::parseOperand($operand, $source, $sql);
        }
        return $constraintSQL;
    }
}

where the line containing the ExtractValue() part is new, the rest is copied from the original Typo3DbQueryParser

a new operand ExtractValue extending the existing PropertyValue is defined as follows:
ExtractValueInterface.php

<?php

namespace VND\MyExtension\Domain\Db;

use TYPO3\CMS\Extbase\Persistence\Generic\Qom\PropertyValueInterface;

interface ExtractValueInterface extends PropertyValueInterface {
    /**
     * Gets the xpath
     *
     * @return string the property name; non-null
     */
    public function getXpath();
}

<?php

namespace VND\MyExtension\Domain\Db;

use TYPO3\CMS\Extbase\Persistence\Generic\Qom\PropertyValue;

class ExtractValue extends PropertyValue implements ExtractValueInterface {
    /**
     * @var string
     */
    protected $xpath;

    /**
     * Constructs this ExtractValue instance
     *
     * @param string $propertyName
     * @param string $selectorName
     * @param string $xpath
     */
    public function __construct($xpath, $propertyName, $selectorName)
    {
        parent::__construct($propertyName, $selectorName);
        $this->xpath = $xpath;
    }

    /**
     * Gets the xpath
     *
     * @return string xpath; non-null
     */
    public function getXpath()
    {
        return $this->xpath;
    }
}

and in Query.php you would have something like

<?php
namespace VND\MyExtension\Xclass\Db;

/**
 * The Query class used to run queries against the database
 *
 * @api
 */
class Query extends \TYPO3\CMS\Extbase\Persistence\Generic\Query {
   /**
     * Returns an equals criterion used for matching objects against a query
     *
     * @param string $xpath xpath to query for
     * @param string $propertyName The name of the property to compare against
     * @param mixed $operand The value to compare with
     * @return \TYPO3\CMS\Extbase\Persistence\Generic\Qom\ComparisonInterface
     * @api
     */
    public function equalsExtractValue($xpath, $propertyName, $operand)
    {
        $comparison = $this->qomFactory->comparison(
                $this->qomFactory->extractValue($xpath, $propertyName, $this->getSelectorName()),
                QueryInterface::OPERATOR_EQUAL_TO,
                $operand
            );

        return $comparison;
    }

and finally in the QueryObjectModelFactory.php you have

namespace VND\MyExtension\Xclass\Db;

class QueryObjectModelFactory extends \TYPO3\CMS\Extbase\Persistence\Generic\Qom\QueryObjectModelFactory {
    /**
     * Evaluates to the value (or values, if multi-valued) of a property in the specified or default selector.
     *
     * @param string $xpath xpath
     * @param string $propertyName the property name; non-null
     * @param string $selectorName the selector name; non-null
     * @return \VND\MyExtension\Domain\Db\ExtractValueInterface the operand; non-null
     * @throws \TYPO3\CMS\Extbase\Persistence\Generic\Exception\RepositoryException if the operation otherwise fails
     */
    public function extractValue($xpath, $propertyName, $selectorName = '')
    {
        return $this->objectManager->get(\VND\MyExtension\Domain\Db\ExtractValue::class, $xpath, $propertyName, $selectorName);
    }
}

Note for developers that want to use this feature in TYPO3 7 or 8:
If you don't want to xclass the QueryObjectModelFactory or Query classes, you can also just xclass the Typo3DbQueryParser as explained above and use the ExtractValue object in your repository directly:

namespace BKW\BkwTiles\Domain\Repository;

use VND\MyExtension\Domain\Db\ExtractValue;

class SampleRepository extends \TYPO3\CMS\Extbase\Persistence\Repository {

  /**
     * @var \TYPO3\CMS\Extbase\Persistence\Generic\Qom\QueryObjectModelFactory
     */
    protected $qomFactory;

    /**
     * @param \TYPO3\CMS\Extbase\Persistence\Generic\Qom\QueryObjectModelFactory $qomFactory
     */
    public function injectQomFactory(\TYPO3\CMS\Extbase\Persistence\Generic\Qom\QueryObjectModelFactory $qomFactory)
    {
        $this->qomFactory = $qomFactory;
    }

  /**
     * Evaluates to the xpath of an xml-value (or values, if multi-valued) of a property in the specified or default selector.
     *
     * @param string $propertyName the property name; non-null
     * @param string $selectorName the selector name; non-null
     * @return \TYPO3\CMS\Extbase\Persistence\Generic\Qom\PropertyValueInterface the operand; non-null
     * @throws \TYPO3\CMS\Extbase\Persistence\Generic\Exception\RepositoryException if the operation otherwise fails
     */
    public function extractValue($xpath, $propertyName, $selectorName = '')
    {
        return $this->objectManager->get(ExtractValue::class, $xpath, $propertyName, $selectorName);
    }

      /**
     * Returns an equals criterion used for matching objects against a query
     *
     * @param string $xpath xpath to query for
     * @param string $propertyName The name of the property to compare against
     * @param mixed $operand The value to compare with
     * @return \TYPO3\CMS\Extbase\Persistence\Generic\Qom\ComparisonInterface
     * @api
     */
    public function equalsExtractValue($xpath, $propertyName, $operand)
    {
        $comparison = $this->qomFactory->comparison(
                $this->qomFactory->extractValue($xpath, $propertyName, $this->getSelectorName()),
                QueryInterface::OPERATOR_EQUAL_TO,
                $operand
            );

        return $comparison;
    }

   /**
     * Returns all something elements on a page, filtered by title
     * @param string $someTitle title to filter for
     * @return array|\TYPO3\CMS\Extbase\Persistence\QueryResultInterface
     * @throws \TYPO3\CMS\Extbase\Persistence\Exception\InvalidQueryException
     */
    public function findFiltered($someTitle) {
      $query->logicalAnd([
        $query->equals('ctype', 'something'),
        $this->equalsExtractValue('//data/sheet[@index="text"]/language[1]/field[@index="title"]/value', 'pi_flexform', $someTitle)
      ]);
    }
}

So, what do you guy think about this feature / approach? It would be nice to see this in the TYPO3 extbase core. It helps handling flexforms a lot and encourages people to use extbase for database abstraction and handling content elements.