selected non-aggregate values must be part of the associated group - forum topic by ptomar

I'm trying to run the following query: 

SELECT 
vtiger_products.productcategory AS "Product Category",
SUM(vtiger_inventoryproductrel.quantity) AS "Line Item Qt" 
FROM bi_batch.vtiger_inventoryproductrel
INNER JOIN bi_batch.vtiger_products
ON vtiger_products.productid = vtiger_inventoryproductrel.productid

getting error - SELECT Failed. 3504: selected non-aggregate values must be part of the associated group
Can someone help?