You can get all discount from TrxTransactionSaleItem table
ManualDiscountAmount: user manual enter discount
SystemDiscountAmount: system get discount from special price
CouponDiscountAmount: discount from coupon
PromotionalDiscountAmount: discount from Promotion
DiscountAmount= ManualDiscountAmount+ ManualDiscountAmount+ CouponDiscountAmount+ PromotionalDiscountAmount
SaleDiscountAmount: the discount from header (TrxTransaction.DiscountAmount) is allocated to each line.
So total discount of each line will be = DiscountAmount+SaleDiscountAmount