How ROWNUM works in pagination query? [duplicate]

Answer 1

I usually write a query like this:

select * 
from 
(
    select a.*, rownum as rn  
    from table_name a
    where rownum <= 110
)
where rn > 100 

Answer 2

You have 4 questions, and all revolve around the usage and functionality of ROWNUM. I will answer each question one-by-one.

Why (this was my first attempt until I search on SO) Select * From Person Where rownum > 100 and rownum < 110; returns 0 rows ?

Nice explanation by Thomas Kyte regarding ROWNUM and pagination here.

A ROWNUM value is assigned to a row after it passes the predicate phase of the query but before the query does any sorting or aggregation. Also, a ROWNUM value is incremented only after it is assigned, which is why the following query will never return a row:

select * 
  from t 
 where ROWNUM > 1;

Because ROWNUM > 1 is not true for the first row, ROWNUM does not advance to 2. Hence, no ROWNUM value ever gets to be greater than 1.

Why there is no simple way to do something like Select ... FROM ... WHERE rownum BETWEEN lowerBound AND upperBound ?

Yes, there is. From Oracle 12c onwards, you could use the new Top-n Row limiting feature. See my answer here.

For example, the below query would return the employees between 4th highest till 7th highest salaries in ascending order:

SQL> SELECT empno, sal
  2  FROM   emp
  3  ORDER BY sal
  4  OFFSET 4 ROWS FETCH NEXT 4 ROWS ONLY;

     EMPNO        SAL
---------- ----------
      7654       1250
      7934       1300
      7844       1500
      7499       1600

SQL>

How to get rid of the r column in the resulting values?

Instead of select *, list the required column names in the outer query. For frequently using the query, creating a view is a simple one time activity.

Alternatively, in SQL*Plus you could use the NOPRINT command. It will not display the column name you don't want to display. However, it would only work in SQL*Plus.

For example,

COLUMN column_name NOPRINT

For example,

SQL> desc dept
 Name                                      Null?    Type
 ----------------------------------------- -------- ------------
 DEPTNO                                             NUMBER(2)
 DNAME                                              VARCHAR2(14)
 LOC                                                VARCHAR2(13)

SQL> COLUMN dname NOPRINT
SQL> COLUMN LOC NOPRINT
SQL> SELECT * FROM dept;

    DEPTNO
----------
        10
        20
        30
        40

SQL>

Does it ensure correct pagination?

Yes, if you write the pagination query correctly.

For example,

SELECT val
FROM   (SELECT val, rownum AS rnum
        FROM   (SELECT val
                FROM   t
                ORDER BY val)
        WHERE rownum <= 8)
WHERE  rnum >= 5;

       VAL
----------
         3
         3
         4
         4

4 rows selected.

SQL>

Or, use the new row limiting feature on 12c as I have shown above.

Few good examples here.

Answer 3

Answer to question 2: In Oracle 12 you can use pagination

select owner, object_name, object_id
from t
order by owner, object_name
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY;