I have to calculate total quantity sold for last 6 months. For example in case of January 2018 , I have to calculate told quantity sold from July - Dec 2017. This total should be grouped by primary key.
You can calculate the range using ADD_MONTHS plus TRUNC:
WHERE datecole BETWEEN Trunc(Add_Months(Current_Date, -6), 'mon')
AND Trunc(Current_Date, 'mon') -1