Create a last url column from current list of hits - mysql

Question

I currently have the following columns:
hit_id, visit_id, timestamp, page_url, page_next
hit_id increments upwards
visit_id is an ID of the visit and unique to each visitor
timestamp is a unix timestamp of the hit
page_url is the page being looked at
page_next is the page that was looked at next
I would like to to add a new column, page_last, where the previous page URL would go into - I should be able to extract this from page_url and page_next. I do not know why I did not create this column in the first place, probably a slight over-site really.
Is there anyway to fill this column using some MySQL trickery? page_last would always be empty on the initial hit on the website (doesn't contain referrer website).

Answer 1

I find the name page_last ambiguous (does it mean the previous page? or this last page on the visit?). I suggest you change it to page_prev.
The following comes close to filling this in, assuming that no one visited the same page multiple times in a visit:
select h.*, hprev.page_url as page_prev
from hits h left outer join
hits hprev
on hprev.page_next = h.page_url and hprev.visit_id = h.visit_id
If that is not true, then you need the most recent one. You can get that using a correlated subquery:
select h.*,
(select h2.page_url
from hits h2
where h2.visit_id = h.visit_id and h2.page_next = h.page_url and
h2.timestamp < h.timestamp
order by timestamp desc
limit 1
) as page_prev
from hits h
Doing the update is a bit tricky in MySQL, because you are not able to directly use the updated table in the update. But, the following trick should work:
update hits
set page_prev = (select page_url
from (select page_url
from hits h2
where h2.visit_id = hits.visit_id and
h2.page_next = hits.page_url and
h2.timestamp < hits.timestamp
order by timestamp desc
limit 1
) h3
)
The trick works because MySQL materializes views, so it actually creates a "temporary table" containing the necessary information for the update.