is there a PRODUCT function like there is a SUM function in Oracle SQL?

DECLARE @a int
SET @a = 1
-- re-assign @a for each row in the result
-- as what @a was before * the value in the row
SELECT @a = @a * amount
FROM theTable

There's a way to do string concat that is similiar:

DECLARE @b varchar(max)
SET @b = ""

SELECT @b = @b + CustomerName
FROM Customers

select exp(sum(ln(col)))
  from table;

edit:

if col always > 0