DB2 query to find average sale for each item 1 year previous

Having some trouble figuring out how to make these query.

In general I have a table with

• sales_ID
• Employee_ID
• sale_date
• sale_price

what I want to do is have a view that shows for each sales item how much the employee on average sells for 1 year previous of the sale_date.

example: Suppose I have this in the sales table

sales_ID    employee_id    sale_date    sale_price
1           Bob            2016/06/10    100
2           Bob            2016/01/01    75
3           Bob            2014/01/01    475
4           Bob            2015/12/01    100
5           Bob            2016/05/01    200
6           Fred           2016/01/01    30
7           Fred           2015/05/01    50

for sales_id 1 record I want to pull all sales from Bob by 1 year up to the month of the sale (so 2015-05-01 to 2016-05-31 which has 3 sales for 75, 100, 200) so the final output would be

sales_ID    employee_id    sale_date    sale_price    avg_sale
1           Bob            2016/06/10    100          125
2           Bob            2016/01/01    75           275
3           Bob            2014/01/01    475          null
4           Bob            2015/12/01    100          475
5           Bob            2016/05/01    200          87.5
6           Fred           2016/01/01    30           50
7           Fred           2015/05/01    50           null

What I tried doing is something like this

select a.sales_ID, a.sale_price, a.employee_ID, a.sale_date, b.avg_price
from sales a
left join (
     select employee_id, avg(sale_price) as avg_price
     from sales 
     where sale_date between Date(VARCHAR(YEAR(a.sale_date)-1) ||'-'|| VARCHAR(MONTH(a.sale_date)-1) || '-01')
                  and Date(VARCHAR(YEAR(a.sale_date)) ||'-'|| VARCHAR(MONTH(a.sale_date)) || '-01') -1 day
    group by employee_id
) b on a.employee_id = b.employee_id

which DB2 doesn't like using the parent table a in the sub query, but I can't think of how to properly write this query. any thoughts?