oracle查询的控制结果

Question

I have a schema like this

 create table sample(id number ,name varchar2(30),mark number);

Now i has to return names of the top three marks. How can i write sql query for this?

If i use max(mark) it will return only maximum and

  select name from sample 

returns all the names!! I tried in many ways but i was unable to control the result to 3 rows.. Please suggest the way to get rid of my problem..

Answer 1

How do you want to handle ties? If Mary gets a mark of 100, Tom gets a mark of 95, and John and Dave both get a mark of 90, what results do you want, for example? Do you want both John and Dave to be returned since they both tied for third? Or do you want to pick one of the two so that the result always has exactly three rows? What happens if Beth also tied for second with a score of 95? Do you still consider John and Dave tied for third place or do you consider them tied for fourth place?

You can use analytic functions to get the top N results though which analytic function you pick depends on how you want to resolve ties.

SELECT id,
       name,
       mark
  FROM (SELECT id,
               name,
               mark,
               rank() over (order by mark desc) rnk
          FROM sample)
 WHERE rnk <= 3

will return the top three rows using the RANK analytic function to rank them by MARK . RANK returns the same rank for people that are tied and uses the standard sports approach to determining your rank so that if two people tie for second, the next competitor is in fourth place, not third. DENSE_RANK ensures that numeric ranks are not skipped so that if two people tie for second, the next row is third. ROW_NUMBER assigns each row a different rank by arbitrarily breaking ties.

If you really want to use ROWNUM rather than analytic functions, you can also do

SELECT id,
       name,
       mark
  FROM (SELECT id,
               name,
               mark
          FROM sample
         ORDER BY mark DESC)
 WHERE rownum <= 3

You cannot, however, have the ROWNUM predicate at the same level as the ORDER BY clause since the predicate is applied before the ordering.

Answer 2

SELECT t2.name FROM
(
SELECT t.*, t.rownum rn
FROM sample t
ORDER BY mark DESC
) t2
WHERE t2.rn <=3