SELECT
  material,
  material_desc,
  extract_date,
  deliv_date,
  SUM(quantity) AS quantity,
  CASE
      WHEN
      (MONTH(deliv_date)=1 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=2 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=3 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=4 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=5 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=6 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=7 AND year(deliv_date)=2018) THEN '6months'

      WHEN
      (MONTH(deliv_date)=1 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=2 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=3 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=4 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=5 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=6 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=7 AND year(deliv_date)=2018 ) OR
      (MONTH(deliv_date)=8 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=9 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=10 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=11 AND year(deliv_date)=2018) OR
      (MONTH(deliv_date)=12 AND year(deliv_date)=2018) THEN '12Months'
and same code for 18 months...
      ELSE NULL END AS Forecast_18_months
      FROM table
      GROUP BY
      material,
      material_desc,
      extract_date,
      deliv_date,
      Forecast_18_months