How to deal with an overflow when multiplying decimals

Maya Opperman wrote:

Is there an way of preventing an overflow error no the following stored procedure calculation

OExtendedPrice = cast((IPrice * IQty) as decimal(18,2));

IQTY decimal(18,3) = 100.000
IPRICE decimal(18,2) = 4,039,250,052,178.13
OEXTENDEDPRICE decimal(18,2) will be 403,925,005,217,813.00 (still within 18 significant
digits, 17 actually)

I tried:

OExtendedPrice = cast((IPrice * IQty) as decimal(18,2));

but it still gives an error.

I know it gives an error because the result is 403,925,005,217,813.00000 before being passed to 403,925,005,217,813.00, but is there any way I can tell it to drop the last 3 decimal places up front?

Ivan Prenosil answers:

I showed the solution on FB Conference 2006, you can obtain correct result by using this "formula":

A × (B+C) = A × B + A × C

i.e. split one of the values into integer and fraction part, and cast intermediate NUMERIC(x,5) to (x,2), e.g.:

IPrice * CAST(IQty AS BIGINT) + CAST(IPrice * (IQty - CAST(IQty AS BIGINT)) AS NUMERIC(18,2))