Question

Consider these two queries:

SELECT *, MAX(age) AS maxAge FROM someTable ORDER BY age ASC;
SELECT *, 'dummyC' AS dummyC FROM someTable ORDER BY age ASC;

The former query returns all rows and all columns of the table, plus an additional dummy column. The later query returns only a single row, that row which has the lowest primary key. Why is that, and how can I work around it? Tested in MySQL 5.1 on some old but stable Debian server.

Answer 1

This is a MySQL extension.

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. 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.

The value you get is indeterminate. You will often get the first row that was inserted into the table, but this is not guaranteed.

If you want the corresponding values from the row containing the highest age then it is better to use a combination of ORDER BY and LIMIT 1:

SELECT *
FROM someTable
ORDER BY age DESC
LIMIT 1;