Joining with the most recently-entered value

Question

I have a table that shows work tickets, basically like this:

TABLE "ticket"
ticket_id INT UNSIGNED AUTO_INCREMENT
created TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ticket_name VARCHAR(64)

Then I have a table showing the possible list of status codes for a ticket:

TABLE "status_ticket"
status_id INT UNSIGNED AUTO_INCREMENT
created TIMESTAMP DEFAULT CURRENT_TIMESTAMP
status_name VARCHAR(64)

And I have one more table that logs the history of the statuses that a ticket has or has had:

TABLE "xref_ticket_status"
xref_id INT UNSIGNED AUTO_INCREMENT
created TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ticket_id INT UNSIGNED NOT NULL
status_id INT UNSIGNED NOT NULL

When the ticket is created, an entry is made in the ticket table for the ticket, and then an entry is made in the xref_ticket_status table assigning a status to the ticket by linking the ticket table to the status_ticket table. When the status changes, a new entry is made in the xref_ticket_status table to reflect the new status. This way, I have a history of each status that a ticket has had and when it was assigned. The most recent entry for any given ticket_id in the xref_ticket_status table is the ticket's current status.

I'm not sure how I would join these three tables together to get a ticket's current status. Essentially, I want to join the ticket table with the xref_ticket_status table where ticket_id matches but for the newest created column in the xref_ticket_status.

Thanks!

Answer 1

This should do the trick:

select ti.ticket_id,ti.ticket_name,
xr.created, st.status_name
from ticket ti
inner join xref_ticket_status xr
on(ti.ticket_id = xr.ticket_id)
inner join status_ticket st
on(xr.status_id = st.status_id)
group by ti.ticket
order by xr.xref_id desc;

===============Update==================

select * from
(select ti.ticket_id,ti.ticket_name,
xr.created, st.status_name
from ticket ti
inner join xref_ticket_status xr
on(ti.ticket_id = xr.ticket_id)
inner join status_ticket st
on(xr.status_id = st.status_id)
group by ti.ticket
order by xr.xref_id desc)cs
order by cs.ticket_name desc; //or asc

Answer 2

The following gets the most recent status_id and ticket_id from xref_ticket_status:

select xts.*
from xref_ticket_status xts
where not exists (select 1
                  from xref_ticket_status xts2
                  where xts2.ticket_id = xts.ticket_id and
                        xts2.created > xts.created
                 );

The logic is: "Get me all rows from xref_ticket_status where there is no row with the same ticket id and a more recent created date". The would be the last date for each ticket. And note that you can also use xref_id for the comparison rather than the date.

The rest is just joining the tables together and choosing your desired columns:

select t.*, ts.*, xts.created
from xref_ticket_status xts join
     ticket t
     on xts.ticket_id = t.ticket_id join
     ticket_status ts
     on xts.status_id = ts.status_id
where not exists (select 1
                  from xref_ticket_status xts2
                  where xts2.ticket_id = xts.ticket_id and
                        xts2.created > xts.created
                 );