public function assembleSubStringSelectExpression(SubStringSelectExpression $expr)
 {
     return $expr->getFunctionName() . '(' . $this->getAssembler()->assembleSegment($expr->getField()) . ',' . $expr->getStartPosition() . ($expr->hasLength() ? ',' . $expr->getLength() : '') . ')' . ($expr->hasAlias() ? ' AS ' . $this->escapeField($expr->getAlias()) : '');
 }