SET @sql = '';
SELECT
    @sql := CONCAT(@sql,if(@sql='','',', '),temp.output)
FROM
(
    SELECT
      DISTINCT
        CONCAT(
         'MAX(IF(pa.fieldname = ''',
          fieldname,
          ''', pa.fieldvalue, NULL)) AS ',
          fieldname
        ) as output
    FROM
        product_additional
) as temp;

SET @sql = CONCAT('SELECT p.id
                    , p.name
                    , p.description, ', @sql, ' 
                   FROM product p
                   LEFT JOIN product_additional AS pa 
                    ON p.id = pa.id
                   GROUP BY p.id');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Таблица product

 id | name     | description   | stock  |

  1 | product1 | first product |    5   | 
  2 | product2 | sec   product |    5   | 

Таблица product_additional

| id | fieldname  | fieldvalue |

  1 | size    | S    |
  1 | height  | 103  |
  2 | size    | L    |
  2 | height  | 13   |
  2 | color   | black|

var
  i: Integer;
begin
  FDConnection1.ResourceOptions.ServerOutput := True;
  FDQuery1.FetchOptions.AutoClose := False;
  FDQuery1.Open('select * from Region; print ''Hello''');
  FDMemTable1.Data := FDQuery1.Data;
  Memo1.Lines.Add(Format('%d rows processed', [FDMemTable1.RecordCount]));
  FDQuery1.NextRecordSet;
  if FDConnection1.Messages <> nil then
    for i := 0 to FDConnection1.Messages.ErrorCount - 1 do
      Memo1.Lines.Add(FDConnection1.Messages[i].Message);
end;

procedure TfReport.btn1Click(Sender: TObject);
begin
     dmReport.fdNext.Active:=False;
     dmReport.fdNext.Active:=True;
     dmReport.FDMemTable1.Data := dmReport.fdNext.Data;
     dmReport.fdNext.NextRecordSet;
     labNstrok.Caption:=IntToStr(dmReport.FDMemTable1.RecordCount);
end;