Oracle Sql query to count time span with certain criteria - oracle

Question

Oracle Sql query , I was trying to count the grand total for time difference that is greater than 2, but when I tried this it just counted all the rows from the query instead of just the rows that have the criteria I was looking for. Anybody have an idea of what I am missing or a better approach . Thanks
This is my query
select DC.CUST_FIRST_NAME,DC.CUST_LAST_NAME,oi.customer_id,oi.order_timestamp,oi.order_timestamp - LAG(oi.order_timestamp) OVER (ORDER BY oi.order_timestamp) AS "Difference(In Days)" ,
(select Count('Elapsed Order Difference')
from demo_orders oi,
demo_customers dc
where OI.CUSTOMER_ID = DC.CUSTOMER_ID
group by 'Elapsed Order Difference'
having count('Elapsed Order Difference') > 3
)Total
from demo_orders oi,
demo_customers dc
where OI.CUSTOMER_ID = DC.CUSTOMER_ID
Results
CUST_FIRST_NAME CUST_LAST_NAME CUSTOMER_ID ORDER_TIMESTAMP Difference(In Days) TOTAL
Eugene Bradley 7 8/14/2013 5:59:11 PM 10
William Hartsfield 2 8/28/2013 5:59:11 PM 14 10
Edward "Butch" OHare 4 9/8/2013 5:59:11 PM 11 10
Edward Logan 3 9/10/2013 5:59:11 PM 2 10
Edward Logan 3 9/20/2013 5:59:11 PM 10 10
Albert Lambert 6 9/25/2013 5:59:11 PM 5 10
Fiorello LaGuardia 5 9/30/2013 5:59:11 PM 5 10
William Hartsfield 2 10/8/2013 5:59:11 PM 8 10
John Dulles 1 10/14/2013 5:59:11 PM 6 10
Eugene Bradley 7 10/17/2013 5:59:11 PM 3 10

Answer 1

This is untested, but I think it might give you what you're after.
with raw_data as (
select
dc.cust_first_name, dc.cust_last_name,
oi.customer_id, oi.order_timestamp,
oi.order_timestamp - LAG(oi.order_timestamp) OVER
(ORDER BY oi.order_timestamp) AS "Difference(In Days)",
case
when oi.order_timestamp - LAG(oi.order_timestamp)
over (ORDER BY oi.order_timestamp) > 2 then 1
else 0
end as gt2
from
demo_orders oi,
demo_customers dc
where
oi.customer_id = dc.customer_id
)
select
cust_first_name, cust_last_name,
customer_id, order_timestamp,
"Difference(In Days)",
sum (gt2) over (partition by 1) as total
from raw_data
When you do Count('Elapsed Order Difference') above, you are counting every row, no matter what. You could have put count ('frog') or count (*) and have gotten the same result. The having count > 3 was already satisfied since the count of all rows was 10.
In general, I'd try to avoid using a scalar for a field in a query as you have in your example. I'm not saying it's never a good idea, but I would argue that there is usually a better way to do it. With 10 rows, you'll hardly notice a performance difference, but as your datasets grow, this can create issues.
Expected output:
fn ln id order date dif total
E B 7 8/14/2014 8
W H 2 8/28/2014 14 8
E O 4 9/8/2014 11 8
E L 3 9/10/2014 2 8
E L 3 9/20/2014 10 8
A L 6 9/25/2014 5 8
F L 5 9/30/2014 5 8
W H 2 10/8/2014 8 8
J D 1 10/14/2014 6 8
E B 7 10/17/2014 3 8