How to group by DESC order

I have the following table called questions:

ID | asker 
1  | Bob
2  | Bob
3  | Marley

I want to select each asker only once and if there are multiple askers with the same name, select the one of the highest id. So, the expected results:

ID | asker 
3  | Marley
2  | Bob

I use the following query:

SELECT * FROM questions GROUP by questions.asker ORDER by questions.id DESC

I get the following result:

ID | asker 
3  | Marley
1  | Bob

So it selects the first 'Bob' it encounters instead of the last one.

Thanks

Answers


If you want the last id for each asker, then you should use an aggregate function:

SELECT max(id) as id, 
   asker
FROM questions 
GROUP by asker 
ORDER by id DESC

The reason why you were getting the unusual result is because MySQL uses an extension to GROUP BY which allows items in a select list to be nonaggregated and not included in the GROUP BY clause. This however can lead to unexpected results because MySQL can choose the values that are returned. (See MySQL Extensions to GROUP BY)

From the MySQL Docs:

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. ... You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, 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. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values the server chooses.

Now if you had other columns that you need to return from the table, but don't want to add them to the GROUP BY due to the inconsistent results that you could get, then you could use a subquery to do so. (Demo)

select 
  q.Id,
  q.asker,
  q.other -- add other columns here
from questions q
inner join
(
  -- get your values from the group by
  SELECT max(id) as id, 
    asker
  FROM questions 
  GROUP by asker 
) m
  on q.id = m.id
order by q.id desc

Normally MySQL allows group by ascending order records only. So we can order records before grouping.

SELECT *
FROM (
  SELECT *
  FROM questions
  ORDER BY id DESC
) AS questions
GROUP BY questions.asker

The records need to be grouped using GROUP BY and MAX() to get the maximum ID for every asker.

SELECT  asker, MAX(ID) ID
FROM    TableName
GROUP   BY asker

OUTPUT

╔════════╦════╗
║ ASKER  ║ ID ║
╠════════╬════╣
║ Bob    ║  2 ║
║ Marley ║  3 ║
╚════════╩════╝

The others are correct about using MAX(ID) to get the results you want. If you're wondering why your query doesn't work, it's because ORDER BY happens after the GROUP BY.


To get every column:

SELECT * FROM questions
WHERE id IN 
(SELECT max(id) as id, asker
FROM questions 
GROUP by asker 
ORDER by id DESC)

Improved version of the answer of @bluefeet.


Im writing this answer because @Taryn's first/shorter alternative in accepted answer works only if you are exactly selecting just columns used in GROUP BY and MAX. User asking question is selecting all columns in table (he used SELECT *). So when you add another 3rd column to table, that column value in query result will be incorrect. You will get mixed values from different table rows. @Taryn's second/longer alternative (using inner join and subquery) works but query is uselessly complicated and is 5 times slower in my use case than my simple alternative below.


Consider table questions:

id | asker 
-----------
1  | Bob
2  | Bob
3  | Marley

Query SELECT max(id) as id, asker FROM questions GROUP BY asker ORDER BY id DESC returns expected:

id | asker 
-----------
3  | Marley
2  | Bob

Now consider another table questions:

id | asker  | other
-------------------
1  | Bob    | 1st
2  | Bob    | 2nd
3  | Marley | 3rd

Query SELECT max(id) as id, asker, other FROM questions GROUP BY asker ORDER BY id DESC returns unexpected:

id | asker  | other
-------------------
3  | Marley | 3rd
2  | Bob    | 1st

... note that value of other for second row of result is incorrect because id=2 comes from second row of table but other=1st comes from first row of table! That is way many users in comments of Taryn's answer reports that this solution does not work.


Possible simple solution when selecting also another columns is to use GROUP BY + DESC:

SELECT id, asker, other FROM questions GROUP BY asker DESC

id | asker  | other
-------------------
3  | Marley | 3rd
2  | Bob    | 2nd

(see demo: https://www.db-fiddle.com/f/esww483qFQXbXzJmkHZ8VT/10)

... but this simple solution has some limitations:

  • Table must be InnoDB (I think it is not problem because you will get better performance and also since MySQL >= 5.5.5 default/preferred storage engine was changed from MyISAM to InnoDB)
  • You have to create index for column which is used in GROUP BY - so asker in this case (I think it is not problem because you will get better performance since index is suitable in this case. GROUP BY usually needs creation of tmp table but when index is available tmp table will not be created which is faster)
  • For MySQL 5.7 and 8.0 is needed to disable SQL mode ONLY_FULL_GROUP_BY (e.g SET SESSION sql_mode = '';) or use ANY_VALUE() on selected columns which are not aggregated to avoid error ER_WRONG_FIELD_WITH_GROUP.
  • Unfortunately MySQL developers removed support of ASC/DESC withing GROUP BY since MySQL 8.0 https://dev.mysql.com/worklog/task/?id=8693 but fortunately there is alternative GROUP BY col1 ORDER BY col1 ASC/DESC:

SELECT id, asker, other FROM questions GROUP BY asker ORDER BY asker DESC

id | asker  | other
-------------------
3  | Marley | 3rd
2  | Bob    | 2nd

(see demo: https://www.db-fiddle.com/f/esww483qFQXbXzJmkHZ8VT/11)

... result is the same as above with GROUP BY ... DESC (do not forget to use InnoDB and create index).


It's because ORDER BY is performed AFTER GROUP BY.

Try this:

SELECT * FROM questions
WHERE id IN 
(
    SELECT max(id) as id
    FROM questions 
    GROUP by asker 
    ORDER by id DESC
)

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