Think\Db\Builder::parseJoin PHP Метод

    protected function parseJoin($join, $options = [])
    {
        $joinStr = '';
        if (!empty($join)) {
            foreach ($join as $item) {
                list($table, $type, $on) = $item;
                $condition = [];
                foreach ((array) $on as $val) {
                    if (strpos($val, '=')) {
                        list($val1, $val2) = explode('=', $val, 2);
                        $condition[] = $this->parseKey($val1, $options) . '=' . $this->parseKey($val2, $options);
                    } else {
                        $condition[] = $val;
                    }
                }
                $table = $this->parseTable($table, $options);
                $joinStr .= ' ' . $type . ' JOIN ' . $table . ' ON ' . implode(' AND ', $condition);
            }
        }
        return $joinStr;
    }