To pivot a table based on a specific event value using Query - sql

Question

I want to make Table A like Table B.
I'd like to see what events the User caused before the Purchase event.
I've used row_number() over (partition by client_id, event_type order by time) and it's simply a pivot. How do I make logic?
Table A
client_id event_type count time
A cart 1 AM 12:00:00
A view 4 AM 12:01:00
A purchase 2 AM 12:05:00
A view 2 AM 12:10:00
B view 3 AM 12:03:00
B purchase 1 AM 12:05:00
B view 2 AM 12:10:00
Table B
client_id view cart purchase
A 4 1 2
A 2 0 0
B 3 0 1
B 2 0 0

Answer 1

Here is a way of doing this, i define a group of events as belonging to a single "session/activity" before purchase using the block grp_split.
Then i get this grouping correctly done in the block x, by replacing null values with the previously not null value using the max(grp) over(partition by client_id order by time1) as grp2.
After that its a matter of pivoting the columns for view,cart and purchase
with data
as (
select 'A' as client_id,'cart' as event_type , 1 as count1, cast('AM 12:00:00' as time) as time1 union all
select 'A' as client_id,'view' as event_type , 4 as count1, cast('AM 12:01:00' as time) as time1 union all
select 'A' as client_id,'purchase' as event_type , 2 as count1, cast('AM 12:05:00' as time) as time1 union all
select 'A' as client_id,'view' as event_type , 2 as count1, cast('AM 12:10:00' as time) as time1 union all
select 'B' as client_id,'view' as event_type , 3 as count1, cast('AM 12:03:00' as time) as time1 union all
select 'B' as client_id,'purchase' as event_type , 1 as count1, cast('AM 12:05:00' as time) as time1 union all
select 'B' as client_id,'view' as event_type , 2 as count1, cast('AM 12:10:00' as time) as time1
)
,grp_split
as(
select case when lag(event_type) over(partition by client_id order by time1)='purchase'
or lag(event_type) over(partition by client_id order by time1) is null
then
row_number() over(partition by client_id order by time1)
end as grp
,*
from data
)
select x.client_id
,max(case when event_type='view' then count1 else 0 end) as view
,max(case when event_type='cart' then count1 else 0 end) as cart
,max(case when event_type='purchase' then count1 else 0 end) as purchase
from (
select *
,max(grp) over(partition by client_id order by time1) as grp2
from grp_split
)x
group by client_id
,grp2
order by client_id
output
+-----------+------+------+----------+
| client_id | view | cart | purchase |
+-----------+------+------+----------+
| A | 4 | 1 | 2 |
| A | 2 | 0 | 0 |
| B | 3 | 0 | 1 |
| B | 2 | 0 | 0 |
+-----------+------+------+----------+
working example
https://dbfiddle.uk/?rdbms=postgres_12&fiddle=aeeb0878b9094e061c469bb0efb7a024