src/I18n/DbMessageSource.php
protected function findAllCategories()
    {
        if (isset($this->_categories)) {
            return resolve($this->_categories);
        }
        return (new Query())->select(['category'])
            ->from($this->sourceMessageTable)
            ->orderBy(['category' => SORT_ASC])
            ->groupBy(['category'])
            ->column($this->db)
            ->then(function($categories) {
                return $this->_categories = $categories;
            });
    }
protected function findAllCategories()
    {
        if (isset($this->_categories)) {
            return resolve($this->_categories);
        }
        return (new Query())->select(['category'])
            ->from($this->sourceMessageTable)
            ->orderBy(['category' => SORT_ASC])
            ->groupBy(['category'])
            ->column($this->db)
            ->then(function($categories) {
                return $this->_categories = $categories;
            });
    }
Find all categories used in message source
@return ExtendedPromiseInterface