Perhaps something like this:

declare @ProductType varchar(???),

@SalesIncrease int;

set @ProductType = 'LEISURE';

set @SalesIncrease = 1000;

with ProductsToUpdate as (

select

Year,

Company,

Product,

ProductType,

LastYearsSales,

ForecastSales

ProductCnt = count(Product) over (partition by ProductType)

from

MyTable

where

ProductType = @ProductType

)

update ProductsToUpdate set

ForecastSales = ForecastSales + (@SalesIncrease/ProductCnt);

You will have to do a lot of testing as you didn't provide us with anything that would allows us to build a test environment to test any code we might provide.

Also, note that the integer division may result in less the requested quantity from being actually added to all products. For example 1000 distributed over 12 products would actually result in a total increase of 996, not 1000.