Math operations on two columns of another sheet using SELECT

[Thomas_McIntosh]

I want to multiple values from two columns on another sheet based on a Select  ID

I found this post that shows exactly how to do what I want, but the syntax doesn't work. 

SUM(SELECT(generic_table_name[column_a*column_b];[id_column]=[_thisrow].[id_column]))

I tried both of these syntaxes and neither work:

SUM(SELECT(Purchases[Price*Quantity],AND(([Product Barcode] = [_ThisRow].[Product Barcode]),ISNOTBLANK([Price]),[Price] > 0))
)

SUM(SELECT((Purchases[Price]*Purchases[Quantity]),AND(([Product Barcode] = [_ThisRow].[Product Barcode]),ISNOTBLANK([Price]),[Price] > 0))
)

I realize that the operation can be performed in a Virtual Column on the referenced table, but for readability I would prefer to do the whole calculation in one formula.

Many thanks 🙂