Cool things cross apply does

select * from

(

select 1 as customer_id, 'herby' as name

union select 2 as customer_id, 'george' as name

union select 3 as customer_id, 'ben' as name

) a

cross apply

(

select customer_id, sale,date,

sum(sale) over(partition by customer_id,month(date)) total_of_cust_month,

sum(sale) over(partition by month(date)) total_of_month

from

(

select 1 as customer_id, 34.67 as sale, '1/1/2011' as date

union select 1 as customer_id, 3464.67 as sale, '2/1/2011' as date

union select 1 as customer_id, 364.67 as sale, '2/5/2011' as date

union select 1 as customer_id, 3374.67 as sale, '3/1/2011' as date

union select 2 as customer_id, 3448.67 as sale, '1/1/2011' as date

union select 2 as customer_id, 3324.67 as sale, '2/1/2011' as date

union select 2 as customer_id, 4.67 as sale, '2/4/2011' as date

union select 2 as customer_id, 3443.67 as sale, '3/1/2011' as date

union select 3 as customer_id, 3344.67 as sale, '1/1/2011' as date

union select 3 as customer_id, 3544.67 as sale, '2/1/2011' as date

union select 3 as customer_id, 3.67 as sale, '2/3/2011' as date

union select 3 as customer_id, 6334.67 as sale, '3/1/2011' as date) as main

where main.customer_id = a.customer_id

) b

cool things

1. we can get multiple rows - subqueries cant
2. we can get multiple columns - subqueries cant