Custom aliases are not treated by where clause you need to use HAVING
for this
SELECT a.id_stock, a.color, a.size, (SUM(IFNULL(a.barang_masuk,0)) - SUM(IFNULL(a.reject,0)) - IFNULL((
SELECT SUM(IFNULL(qty,0)) AS SUMqty FROM transaksi WHERE transaksi.id_stock = a.id_stock GROUP BY id_stock
),0)) AS sold FROM stock a
HAVING a.no_barang='10' AND sold>0
GROUP BY a.no_barang, a.color, a.size
ORDER BY color, size ASC
For the problem i guess you cannot use having before group by you can use subselect
SELECT q.* FROM (
SELECT a.id_stock, a.color, a.size, (SUM(IFNULL(a.barang_masuk,0)) - SUM(IFNULL(a.reject,0)) - IFNULL((
SELECT SUM(IFNULL(qty,0)) AS SUMqty FROM transaksi WHERE transaksi.id_stock = a.id_stock GROUP BY id_stock
),0)) AS sold FROM stock a
HAVING a.no_barang='10' AND sold>0
/* you can use where and having both in same query like WHERE a.no_barang='10' HAVING sold>0 */
ORDER BY color, size ASC ) q GROUP BY q.no_barang, q.color, q.size
EDIT
SELECT a.id_stock, a.color, a.size, (SUM(IFNULL(a.barang_masuk,0)) - SUM(IFNULL(a.reject,0)) - IFNULL((
SELECT SUM(IFNULL(qty,0)) AS SUMqty FROM transaksi WHERE transaksi.id_stock = a.id_stock GROUP BY id_stock
),0)) AS sold FROM stock a
WHERE a.no_barang='10'
GROUP BY a.no_barang, a.color, a.size
HAVING sold>0
ORDER BY color, size ASC
Unknown Column In Where Clause
How to use GROUP BY after the Having clause Mysql