select from two tables ordering by one row from second table

Question

I have two tables, tickets and ticket_updates

There are multiple rows in ticket_updates for each one row in tickets

how can i select from tickets ordering by datetime in ticket_updates

i want to order by datetime in ticket_updates from the earliest datetime (DESC)

i thought the following may work, however it takes too long to run in PHPMyAdmin and i get an internal server error:

SELECT t.ticketnumber, t.subject, t.contact_name FROM tickets t JOIN ticket_updates tu on t.ticketnumber = tu.ticketnumber group by tu.ticketnumber order by tu.datetime DESC

Answer 1

You could do something like this:

select t.*
from tickets t
order by (select min(datetime) from ticket_updates tu where tu.ticketid = t.ticketid);

EDIT:

You can also try with a group by :

select t.*
from tickets t join
     (select ticketid, min(datetime) as mindt
      from ticket_updates tu
      group by ticketid
     ) tu
     on tu.ticketid = t.ticketid
group by tu.mindt desc;

Or, the original query can take advantage of an index on ticket_updates(ticketid, datetime) .

Answer 2

I'd think you'd want something like this, which is pretty much what you'd posted after the edit. If it is running too slow you might need to run an EXPLAIN for the query and add an appropriate index. Either to datetime or potentially a compound key to ticketnumber/datetime as Gordon suggested.

SELECT t.ticketnumber, t.subject, t.contact_name FROM ticket_updates tu
INNER JOIN tickets t ON t.ticketnumber = tu.ticketnumber
GROUP BY tu.ticketnumber
ORDER BY tu.datetime DESC

or

SELECT t.ticketnumber, t.subject, t.contact_name FROM ticket_updates tu
INNER JOIN tickets t using(ticketnumber)
GROUP BY tu.ticketnumber
ORDER BY tu.datetime DESC