ORACLE 12C-使用ROWNUM进行SELECT查询

Question

I have to get second five (6-10) best salaries records from sorted table using ROWNUM.

Using ROWNUM is necessary.

When I execute query:

SELECT ROWNUM AS position, name, salary
    FROM (SELECT name, salary
            FROM employees
            ORDER BY salary DESC)
    WHERE ROWNUM <= 10;

I get a first 10 best records.

And now when I try execute query:

    SELECT ROWNUM AS position, name, salary
        FROM (SELECT name, salary
                FROM employees
                ORDER BY salary DESC)
        WHERE ROWNUM >= 6 AND ROWNUM <= 10;

I get a empty table. Why doesn't it work?

Answer 1

As explained in the documentation , rownum is evaluated as the rows are fetched. If you never fetch the first row, you never get to the second. Hence, no rows are fetched:

Conditions testing for ROWNUM values greater than a positive integer are always false. For example, this query returns no rows:

 SELECT * FROM employees WHERE ROWNUM > 1; 

But, more importantly, you are using Oracle 12C. So, use fetch first instead of rownum . This has multiple advantages. Besides being standard SQL, you don't need a subquery:

SELECT name, salary
FROM employees
ORDER BY salary DESC
FETCH FIRST 10 ROWS ONLY;

And for your second:

SELECT name, salary
FROM employees
ORDER BY salary DESC
OFFSET 5 ROWS
FETCH FIRST 5 ROWS ONLY;

Answer 2

I write in queston that using ROWNUM is necessary, because it's academic task.

In such a case use a subquery

SELECT name, salary
FROM (
  SELECT name, salary, ROWNUM as my_rownum
  FROM employees
  ORDER BY salary DESC
)
WHERE my_rownum BETWEEN 6 AND 10

Answer 3

您可以使用以下查询并尝试...。

SELECT name,salary from ( SELECT name,salary, DENSE_RANK() OVER (ORDER BY salary DESC) as rn from employees ) where rn between 6 and 10;