SQL - unique users who are visiting for the first time - sql

Question

Given following table visitorLog, write a SQL to find the following by date.
Total_Visitors
VisitorGain - compare to previous day
VisitorLoss - compare to previous day
Total_New_Visitors - unique users who are visiting for the first time
visitorLog :
*----------------------*
| Date Visitor |
*----------------------*
| 01-Jan-2011 V1 |
| 01-Jan-2011 V2 |
| 01-Jan-2011 V3 |
| 02-Jan-2011 V2 |
| 03-Jan-2011 V2 |
| 03-Jan-2011 V4 |
| 03-Jan-2011 V5 |
*----------------------*
Expected output:
*---------------------------------------------------------------------*
| Date Total_Visitors VisitorGain VisitorLoss Total_New_Visitors |
*---------------------------------------------------------------------*
| 01-Jan-2011 3 3 0 3 |
| 02-Jan-2011 1 0 2 0 |
| 03-Jan-2011 3 2 0 2 |
*---------------------------------------------------------------------*
Here is my SQL and SLQ fiddle.
with cte as
(
select
date,
total_visitors,
lag(total_visitors) over (order by date) as prev_visitors,
row_number() over (order by date ) as rnk
from
(
select
*,
count(visitor) over (partition by date) as total_visitors
from visitorLog
) val
group by
date,
total_visitors
),
cte2 as
(
select
date,
sum(case when rnk = 1 then 1 else 0 end) as total_new_visitors
from
(
select
date,
visitor,
row_number() over (partition BY visitor order by date) as rnk
from visitorLog
) t
group by
date
)
select
c.date,
sum(total_visitors) as total_visitors,
sum(
case
when rnk = 1 then total_visitors
when (rnk > 1 and prev_visitors < total_visitors) then (total_visitors - prev_visitors)
else
0
end
)visitorGain,
sum(
case
when rnk = 1 then 0
when prev_visitors > total_visitors then (prev_visitors - total_visitors)
else
0
end
) as visitorLoss,
sum(total_new_visitors) as total_new_visitors
from cte c
join cte2 c2
on c.date = c2.date
group by
c.date
order by
c.date
My solution is working as expected but I am wondering if I am missing any any edge cases here which may break my logic. any help would be great.

Answer 1

This logic does what you want:
select date, count(*) as num_visitor,
greatest(count(*) - lag(count(*)::int, 1, 0) over (order by date), 0) as visitor_gain,
greatest(lag(count(*)::int, 1, 0) over (order by date) - count(*), 0) as visitor_loss,
count(*) filter (where seqnum = 1) as num_new_visitors
from (select vl.*,
row_number() over (partition by visitor order by date) as seqnum
from visitorLog vl
) vl
group by date
order by date
Here is a db<>fiddle.

Answer 2

I would use window functions and aggregation:
select
date,
count(*) no_visitor,
count(*) - lag(count(*), 1, 0) over(partition by date) no_visitor_diff,
count(*) filter(where rn = 1) no_new_visitors
from (
select t.*, row_number() over(partition by visitor order by date) rn
from visitorLog
) t
group by date
order by date
The subquery ranks the visits of each customer using row_number() (the first visit of each customer gets row number 1). Then, the outer query aggregates by date, and uses lag() to get the visitor count of the "previous" day.
I don't really see the point to have two distinct columns for the difference of visitors compared to the last day, so this gives you a single column, with a value that's either positive or negative depending whether customers were gained or lost.
If you really want two columns, then:
greatest(count(*) - lag(count(*), 1, 0) over(partition by date), 0) visitor_gain,
- least(count(*) - lag(count(*), 1, 0) over(partition by date), 0) visitor_loss