Невозможно назначить переменную в select (процедура хранения)

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ALTER procedure [dbo].[spDepartmentWorkingSelectAll]

as
set nocount on
declare @SAM float
declare @factor_error float
select [id],
    [date_working],
    [department_id],
    (select top 1 departments.name from departments where departments.id = department_id) as department_name,
    [employee_count],
    [product_quantity],
    [real_time],
    [labor_unit],
    [product_id],
    (select top 1 products.code from products where products.id = [product_id]) as product_code,
    [product_quantity_error],
    @SAM = dbo.fnSam([product_id]), -- SAM
    @SAM as sam,
    @factor_error = dbo.fnFactorError([product_id],[product_quantity],[product_quantity_error]), -- factor error
    @factor_error as factor_error
    (@SAM * 60)/2 as real_output ,
    @SAM * @factor_error as reward_unit
    ((@SAM + @factor_error) * 8)/32 as expected_output
from [department_working]

Что я уже пробовал:

я хочу назначить переменную в select stament для работы, но моя процедура хранения в sql server 2008 не может быть запущена. все, Пожалуйста, помогите мне. спасибо

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