MySQL Group By and Order By

Question

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

User · Answer

I struggled with both these approaches for more complex queries than those shown, because the subquery approach was horribly ineficient no matter what indexes I put on, and because I couldn't get the outer self-join through Hibernate

The best (and easiest) way to do this is to group by something which is constructed to contain a concatenation of the fields you require and then to pull them out using expressions in the SELECT clause. If you need to do a MAX() make sure that the field you want to MAX() over is always at the most significant end of the concatenated entity.

The key to understanding this is that the query can only make sense if these other fields are invariant for any entity which satisfies the Max(), so in terms of the sort the other pieces of the concatenation can be ignored. It explains how to do this at the very bottom of this link. http://dev.mysql.com/doc/refman/5.0/en/group-by-hidden-columns.html

If you can get am insert/update event (like a trigger) to pre-compute the concatenation of the fields you can index it and the query will be as fast as if the group by was over just the field you actually wanted to MAX(). You can even use it to get the maximum of multiple fields. I use it to do queries against multi-dimensional trees expresssed as nested sets.

User · Answer

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 · Answer

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  lt  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  lt  next id

User · Answer

As pointed in a reply already  the current answer is wrong  because the GROUP BY arbitrarily selects the record from the window   If one is using MySQL 5 6  or MySQL 5 7 with ONLY FULL GROUP BY  the correct  deterministic  query is   SELECT incomingEmails     FROM       SELECT fromEmail  MAX timestamp   timestamp      FROM incomingEmails     GROUP BY fromEmail     filtered incomingEmails   JOIN incomingEmails USING  fromEmail  timestamp  GROUP BY fromEmail  timestamp   In order for the query to run efficiently  proper indexing is required   Note that for simplification purposes  I ve removed the LOWER    which in most cases  won t be used

User · Answer

Do a GROUP BY after the ORDER BY by wrapping your query with the GROUP BY like this   SELECT t   FROM  SELECT   FROM table ORDER BY time DESC  t GROUP BY t from

User · Answer

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  quot 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  quot  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

[mysql] MySQL "Group By" and "Order By"

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