Maykonn Maykonn - 4 years ago 112
SQL Question

MySQL wrong results with GROUP BY and ORDER BY

I have a table

user_comission_configuration_history
and I need to select the last Comissions configuration from a
user_id
.

Tuples:

enter image description here

I'm trying with many queries, but, the results are wrong. My last SQL:

SELECT *
FROM(
SELECT * FROM user_comission_configuration_history
ORDER BY on_date DESC
) AS ordered_history
WHERE user_id = 408002
GROUP BY comission_id


The result of above query is:

enter image description here

But, the correct result is:

id user_id comission_id value type on_date
24 408002 12 0,01 PERCENTUAL 2014-07-23 10:45:42
23 408002 4 0,03 CURRENCY 2014-07-23 10:45:41
21 408002 6 0,015 PERCENTUAL 2014-07-23 10:45:18


What is wrong in my SQL?

Answer Source

This is your query:

SELECT *
FROM (SELECT *
      FROM user_comission_configuration_history
      ORDER BY on_date DESC
     ) AS ordered_history
WHERE user_id = 408002
GROUP BY comission_id;

One major problem with your query is that it uses a MySQL extension to group by that MySQL explicitly warns against. The extension is the use of other columns in the in theselect that are not in the group by or in aggregation functions. The warning (here) is:

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. 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.

So, the values returned in the columns are indeterminate.

Here is a pretty efficient way to get what you want (with "comission" spelled correctly in English):

SELECT *
FROM user_commission_configuration_history cch
WHERE NOT EXISTS (select 1
                  from user_commission_configuration_history cch2
                  where cch2.user_id = cch.user_id and
                        cch2.commission_id = cch.commission_id and
                        cch2.on_date > cch.on_date
                 ) AND
      cch.user_id = 408002;
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