Oracle SQL - How to Retrieve highest 5 values of a column

Question

How do you write a query where only a select number of rows are returned with either the highest or lowest column value.

i.e. A report with the 5 highest salaried employees?

Answer 1

Try this one:

SELECT * FROM 
    (SELECT field1, field2 FROM fields order by field1 desc) 
where rownum < 5

Also take a look on this resource for a more detailed description on how rownum works.

Answer 2

The best way to do this is with analytic functions, RANK() or DENSE_RANK() ...

SQL> select * from (
  2        select empno
  3               , sal
  4               , rank() over (order by sal desc) as rnk
  5        from emp)
  6  where rnk <= 5
  7  /

     EMPNO        SAL        RNK
---------- ---------- ----------
      7839       5000          1
      7788       3000          2
      7902       3000          2
      7566       2975          4
      8083       2850          5
      7698       2850          5

6 rows selected.

SQL>

DENSE_RANK() compresses the gaps when there is a tie:

SQL> select * from (
  2        select empno
  3               , sal
  4               , dense_rank() over (order by sal desc) as rnk
  5        from emp)
  6  where rnk <= 5
  7  /

     EMPNO        SAL        RNK
---------- ---------- ----------
      7839       5000          1
      7788       3000          2
      7902       3000          2
      7566       2975          3
      8083       2850          4
      7698       2850          4
      8070       2500          5

7 rows selected.

SQL>

Which behaviour you prefer depends upon your business requirements.

Avoid using solutions based on row number unless you want to arbitrarily truncate your resultset in the event of a tie. There is a difference between asking for the five highest values and the first five records sorted by high values

Answer 3

Oracle 9i+ provides analytic functions:

• ROW_NUMBER
• RANK
• DENSE_RANK

All require the use of the OVER clause, which allows PARTITION BY and ORDER BY clauses to properly tune the ROW_NUMBER/RANK/DENSE_RANK value returned.

Prior to 9i, the only option was to work with ROWNUM - which incidentally is faster than using ROW_NUMBER (link).