How to do sum of sum in mysql query

Just modify GROUP BY, adding WITH ROLLUP:

SELECT a.id AS supplier, sum( processed_weight ) AS total_qtyFROM supplier_inward a  INNER JOIN warehouseb ON a.id = b.supplierWHERE a.master_product_id = '38'GROUP BY b.supplier  WITH ROLLUP

Output:

supplier    total_qty12046       475.0012482        99.00NULL        574.00

how about this:

SELECT SUM(iQuery.total_qty) as iTotalFROM    (SELECT a.id AS supplier, sum( processed_weight ) AS total_qty    FROM supplier_inward a    INNER JOIN warehouseb ON a.id = b.supplier    WHERE a.master_product_id = '38'    GROUP BY b.supplier) as iQuery

try

SELECT sum( processed_weight ) AS total_qtyFROM supplier_inward aINNER JOIN warehouseb ON a.id = b.supplierWHERE a.master_product_id = '38'

EDIT 2 - AFTER comment from OP changing the result structure:

For an additional column try:

SELECT X.supplier,X.total_qty,(SELECT sum( processed_weight )  FROM supplier_inward a INNER JOIN warehouseb ON a.id = b.supplier WHERE a.master_product_id = '38') AS totalqFROM(SELECT a.id AS supplier, sum( processed_weight ) AS total_qty, FROM supplier_inward aINNER JOIN warehouseb ON a.id = b.supplierWHERE a.master_product_id = '38'GROUP BY b.supplier) AS X

For an additonal row:

SELECT a.id AS supplier, sum( processed_weight ) AS total_qtyFROM supplier_inward aINNER JOIN warehouseb ON a.id = b.supplierWHERE a.master_product_id = '38'GROUP BY b.supplierUNION ALLSELECT null, X.total_qtyFROM( SELECT sum( processed_weight ) AS total_qtyFROM supplier_inward aINNER JOIN warehouseb ON a.id = b.supplierWHERE a.master_product_id = '38' ) AS X