I am not sure about your table structure or your requirements . I have shown you the syntax if that is what you are looking for.
SELECT
m.mon,
m.monthnames,
d.Outlet_Name,
e.category_Name,
f.Department_Name,
b.Item_Name,
SUM(a.Item_Qty) AS Qty,
SUM(a.Net_amount+a.Item_tax1) AS NetAmount
FROM (
SELECT 1 AS mon, 'Jan' AS monthnames
UNION
SELECT 2, 'Feb'
UNION
SELECT 3, 'Mar'
UNION
SELECT 4, 'Apr'
UNION
SELECT 5, 'May'
UNION
SELECT 6, 'jun'
UNION
SELECT 7, 'july'
UNION
SELECT 8, 'Aug'
UNION
SELECT 9, 'Sep'
UNION
SELECT 10, 'Oct'
UNION
SELECT 11, 'NoV'
UNION
SELECT 12, 'Dec'
) AS m
LEFT JOIN KOT_Items a
ON MONTH(a.tran_date) = m.mon
LEFT JOIN Item_Master b
ON a.Main_Item_Code=b.Item_Code
LEFT JOIN KOT_Main c
ON a.ref_no=c.ref_no
LEFT JOIN Outlet d
ON d.Outlet_id = c.outlet_id
LEFT JOIN Category_Master e
ON e.Category_Code=b.Category_Code
LEFT JOIN Department_Master f
ON e.Category_Code =f.Category_Code
GROUP BY
m.mon,
m.monthnames,
d.Outlet_Name,
e.category_Name,
f.Department_Name,
b.Item_Name