Student with maximun calsses

Question

I have a question, can you guys plz look into this.
Two tables student and class. I want to pull the data for the student with maximum number of classes
I am using
select student_name, count(*)
from student, class
group by student_name
The above query will give me all the students and there number of classes.
I only want to get the name of the student with maximum number of classes(only 1 row) and i cant use rownum.
Can anyone please look into this and throw some light on how to get the query.

Answer 1

Salim Chelabi  wrote:
YesWell, ok, one table scan, but one more step - VIEW - according to the explain plan. Still better to avoid the subquery.
SQL> explain plan for
  2  select max(dname) keep (dense_rank last order by count(*)) deptno
  3  from   dept, emp
  4  where  dept.deptno=emp.deptno
  5  group by dname;
Explained.
SQL> select * from table(dbms_xplan.display);
Plan hash value: 4041873517
| Id  | Operation                      | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT               |         |     1 |    16 |     7  (29)| 00:00:01 |
|   1 |  SORT AGGREGATE                |         |     1 |    16 |     7  (29)| 00:00:01 |
|   2 |   HASH GROUP BY                |         |     1 |    16 |     7  (29)| 00:00:01 |
|   3 |    MERGE JOIN                  |         |    14 |   224 |     6  (17)| 00:00:01 |
|   4 |     TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    52 |     2   (0)| 00:00:01 |
|   5 |      INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  6 |     SORT JOIN                  |         |    14 |    42 |     4  (25)| 00:00:01 |
|   7 |      TABLE ACCESS FULL         | EMP     |    14 |    42 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   6 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")
       filter("DEPT"."DEPTNO"="EMP"."DEPTNO")
20 rows selected.
SQL> explain plan for
  2  select max(dname) keep (dense_rank last order by ct) deptno
  3  from   (select dept.dname, count(*) ct
  4          from   dept, emp
  5          where  dept.deptno=emp.deptno
  6          group by dname);
Explained.
SQL> select * from table(dbms_xplan.display);
Plan hash value: 3958887906
| Id  | Operation                       | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT                |         |     1 |    22 |     7  (29)| 00:00:01 |
|   1 |  SORT AGGREGATE                 |         |     1 |    22 |            |          |
|   2 |   VIEW                          |         |     4 |    88 |     7  (29)| 00:00:01 |
|   3 |    HASH GROUP BY                |         |     4 |    64 |     7  (29)| 00:00:01 |
|   4 |     MERGE JOIN                  |         |    14 |   224 |     6  (17)| 00:00:01 |
|   5 |      TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    52 |     2   (0)| 00:00:01 |
|   6 |       INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  7 |      SORT JOIN                  |         |    14 |    42 |     4  (25)| 00:00:01 |
|   8 |       TABLE ACCESS FULL         | EMP     |    14 |    42 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   7 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")
       filter("DEPT"."DEPTNO"="EMP"."DEPTNO")
21 rows selected.
SQL>Nicolas.