discount value

SELECT name, (
    SELECT SUM(qty * (
        IF(ifnull(base_ref_rate, 0)=0, ifnull(basic_rate, 0), ifnull(base_ref_rate, 0)) - ifnull(basic_rate, 0))
    )
    FROM `tabSales Invoice Item`
    WHERE `tabSales Invoice Item`.parent = `tabSales Invoice`.name
) AS discount_value
FROM `tabSales Invoice` WHERE docstatus=1