MYSQL shows incorrect rows when using GROUP BY
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13-09-2019 - |
Question
I have two tables:
article('id', 'ticket_id', 'incoming_time', 'to', 'from', 'message')
ticket('id', 'queue_id')
where tickets represent a thread of emails between support staff and customers, and articles are the individual messages that compose a thread.
I'm looking to find the article with the highest incoming time (expressed as a unix timestamp) for each ticket_id, and this is the query I'm currently using:
SELECT article.* , MAX(article.incoming_time) as maxtime
FROM ticket, article
WHERE ticket.id = article.ticket_id
AND ticket.queue_id = 1
GROUP BY article.ticket_id
For example,
:article:
id --- ticket_id --- incoming_time --- to ------- from ------- message --------
11 1 1234567 help@ client@ I need help...
12 1 1235433 client@ help@ How can we help?
13 1 1240321 help@ client@ Want food!
...
:ticket:
id --- queue_id
1 1
...
But the result looks to be the row with the smallest article id instead of what I'm looking for which is the article with the highest incoming time.
Any advice would be greatly appreciated!
Solution
This is a classic hurdle that most MySQL programmers bump into.
- You have a column
ticket_id
that is the argument toGROUP BY
. Distinct values in this column define the groups. - You have a column
incoming_time
that is the argument toMAX()
. The greatest value in this column over the rows in each group is returned as the value ofMAX()
. - You have all other columns of table article. The values returned for these columns are arbitrary, not from the same row where the
MAX()
value occurs.
The database cannot infer that you want values from the same row where the max value occurs.
Think about the following cases:
There are multiple rows where the same max value occurs. Which row should be used to show the columns of
article.*
?You write a query that returns both the
MIN()
and theMAX()
. This is legal, but which row shouldarticle.*
show?SELECT article.* , MIN(article.incoming_time), MAX(article.incoming_time) FROM ticket, article WHERE ticket.id = article.ticket_id AND ticket.queue_id = 1 GROUP BY article.ticket_id
You use an aggregate function such as
AVG()
orSUM()
, where no row has that value. How is the database to guess which row to display?SELECT article.* , AVG(article.incoming_time) FROM ticket, article WHERE ticket.id = article.ticket_id AND ticket.queue_id = 1 GROUP BY article.ticket_id
In most brands of database -- as well as the SQL standard itself -- you aren't allowed to write a query like this, because of the ambiguity. You can't include any column in the select-list that isn't inside an aggregate function or named in the GROUP BY
clause.
MySQL is more permissive. It lets you do this, and leaves it up to you to write queries without ambiguity. If you do have ambiguity, it selects values from the row that is physically first in the group (but this is up to the storage engine).
For what it's worth, SQLite also has this behavior, but it chooses the last row in the group to resolve the ambiguity. Go figure. If the SQL standard doesn't say what to do, it's up to the vendor implementation.
Here's a query that can solve your problem for you:
SELECT a1.* , a1.incoming_time AS maxtime
FROM ticket t JOIN article a1 ON (t.id = a1.ticket_id)
LEFT OUTER JOIN article a2 ON (t.id = a2.ticket_id
AND a1.incoming_time < a2.incoming_time)
WHERE t.queue_id = 1
AND a2.ticket_id IS NULL;
In other words, look for a row (a1
) for which there is no other row (a2
) with the same ticket_id
and a greater incoming_time
. If no greater incoming_time
is found, the LEFT OUTER JOIN returns NULL instead of a match.
OTHER TIPS
SELECT a1.* FROM article a1
JOIN
(SELECT MAX(a2.incoming_time) AS maxtime
FROM article a2
JOIN ticket ON (a2.ticketid=ticket.id)
WHERE ticket.queue_id=1) xx
ON (a1.incoming_time=xx.maxtime);