MySQL "Group By" and "Order By"

user_55732 0
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I want to be able to select a bunch of rows from a table of e-mails and group them by the from sender. My query looks like this:

SELECT 
    `timestamp`, `fromEmail`, `subject`
FROM `incomingEmails` 
GROUP BY LOWER(`fromEmail`) 
ORDER BY `timestamp` DESC

The query almost works as I want it — it selects records grouped by e-mail. The problem is that the subject and timestamp don't correspond to the most recent record for a particular e-mail address.

For example, it might return:

fromEmail: john@example.com, subject: hello
fromEmail: mark@example.com, subject: welcome

When the records in the database are:

fromEmail: john@example.com, subject: hello
fromEmail: john@example.com, subject: programming question
fromEmail: mark@example.com, subject: welcome

If the "programming question" subject is the most recent, how can I get MySQL to select that record when grouping the e-mails?

aggregate-functions group-by sql-order-by mysql sql
6 Answers

A simple solution is to wrap the query into a subselect with the ORDER statement first and applying the GROUP BY later:

SELECT * FROM ( 
    SELECT `timestamp`, `fromEmail`, `subject`
    FROM `incomingEmails` 
    ORDER BY `timestamp` DESC
) AS tmp_table GROUP BY LOWER(`fromEmail`)

This is similar to using the join but looks much nicer.

Using non-aggregate columns in a SELECT with a GROUP BY clause is non-standard. MySQL will generally return the values of the first row it finds and discard the rest. Any ORDER BY clauses will only apply to the returned column value, not to the discarded ones.

IMPORTANT UPDATE Selecting non-aggregate columns used to work in practice but should not be relied upon. Per the MySQL documentation "this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate."

As of 5.7.5 ONLY_FULL_GROUP_BY is enabled by default so non-aggregate columns cause query errors (ER_WRONG_FIELD_WITH_GROUP)

As @mikep points out below the solution is to use ANY_VALUE() from 5.7 and above

See http://www.cafewebmaster.com/mysql-order-sort-group https://dev.mysql.com/doc/refman/5.6/en/group-by-handling.html https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_any-value

user_5528710

Here's one approach:

SELECT cur.textID, cur.fromEmail, cur.subject, 
     cur.timestamp, cur.read
FROM incomingEmails cur
LEFT JOIN incomingEmails next
    on cur.fromEmail = next.fromEmail
    and cur.timestamp < next.timestamp
WHERE next.timestamp is null
and cur.toUserID = '$userID' 
ORDER BY LOWER(cur.fromEmail)

Basically, you join the table on itself, searching for later rows. In the where clause you state that there cannot be later rows. This gives you only the latest row.

If there can be multiple emails with the same timestamp, this query would need refining. If there's an incremental ID column in the email table, change the JOIN like:

LEFT JOIN incomingEmails next
    on cur.fromEmail = next.fromEmail
    and cur.id < next.id
user_505520

According to SQL standard you cannot use non-aggregate columns in select list. MySQL allows such usage (uless ONLY_FULL_GROUP_BY mode used) but result is not predictable.

ONLY_FULL_GROUP_BY

You should first select fromEmail, MIN(read), and then, with second query (or subquery) - Subject.

user_1024840
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