SQL Query: Return Max value record of a Group

Question

I have a sample table with similar structure & data as shown below:

+------+---------+-------------+------------+
| S_ID | S_NAME  | SUBJECT     | MARK_V         


        

Answer 1

Analytic function ROW_NUMBER can be used to group rows by S_NAME (as you want to get maximum mark per student), and sort marks in descending order so that the max value raises to the top (i.e. gets row number = 1).

Then select rows with that row number value.

SQL> with test (s_id, s_name, subject, mark_value) as
  2    (select 1, 'stud', 'sub_1'  , 50 from dual union all
  3     select 2, 'stud', 'sub_2'  , 60 from dual union all
  4     select 3, 'stud', 'sub_3'  , 70 from dual union all
  5     select 4, 'stud_1', 'sub_1', 40 from dual union all
  6     select 5, 'stud_1', 'sub_2', 50 from dual union all
  7     select 6, 'stud_2', 'sub_2', 40 from dual
  8    )
  9  select s_id, s_name, subject, mark_value
 10  from (select s_id, s_name, subject, mark_value,
 11               row_Number() over (partition by s_name order by mark_value desc) rn
 12        from test
 13       )
 14  where rn = 1;

      S_ID S_NAME SUBJE MARK_VALUE
---------- ------ ----- ----------
         3 stud   sub_3         70
         5 stud_1 sub_2         50
         6 stud_2 sub_2         40

SQL>

---

if your database version doesn't support analytic functions, there's another option which isn't that good as it selects from the same table twice. You won't notice the difference if there aren't that many rows in a table, but performance will suffer on large data sets.

 
  9  select s_id, s_name, subject, mark_value
 10  from test
 11  where (s_name, mark_value) in (select s_name, max(mark_value) max_mark
 12                                 from test
 13                                 group by s_name);

      S_ID S_NAME SUBJE MARK_VALUE
---------- ------ ----- ----------
         3 stud   sub_3         70
         5 stud_1 sub_2         50
         6 stud_2 sub_2         40

SQL>