分组依据：如果我不想对列执行聚合函数怎么办？

Question

I have a table, Students, with the following columns:

________________________________________________
| id   |  name  |  class  |  date_registrered  | 
------------------------------------------------

I want to select one row for every unique class, and only the row with the largest value in date_registrered, ie I want to select the latest registrered Student for every class, including all the data for that one.

I tried:

SELECT id, name, class, MAX(date_registrered)
FROM Students
GROUP BY class;

I get the following error:

Column 'Students.id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

This question on SO adresses a simplified version of this issue. However, the example is for 2 columns only.

I only want to group by class, and I only want to perform an aggregate function on date_registrered. I also want to display all the other columns for the row with the max date_registrered for every class.

Do you know how to fix it?

Answer 1

use ROW_NUMBER()

SELECT *
FROM ( SELECT id, name, class, date_registrered
              ROW_NUMBER() OVER (partition by class ORDER BY date_registrered DESC) rn
       FROM Students
     ) T
WHERE T.rn = 1

Answer 2

The error message explains your issue very well, you can't perform an aggregation on one column, and not use the rest in the GROUP BY . In this case, you'll want to use something like ROW_NUMBER :

WITH CTE AS
(
    SELECT  id,
            name,
            class,
            date_registered,
            RN = ROW_NUMBER() OVER(PARTITION BY class ORDER BY date_registrered DESC)
    FROM students
)
SELECT  id,
        name,
        class,
        date_registered
FROM CTE
WHERE RN = 1;