So I want to select a range of rows in an Oracle DB. I need to do this because I have millions of rows in the table and I want to paginate results to the user (if you know another way to do this on the client side, I'm using JavaFX if it matters but I don't think it's a good idea to send all the datas over the network to paginate them on the client side).
So after reading this post: SQL ROWNUM how to return rows between a specific range, I have the following query:
Select * From (Select t.*, rownum r from PERSON t) Where r > 100 and r < 110;
The 100
and 110
are just example. In the application, I just ask for the lower bound and add a size of 10_000 to fetch the next 10_000 rows.
Now the rownum column appears in the result and I don't want to see it. As I'm not very experienced with SQL, here's my questions:
Why (this was my first attempt until I search on SO) Select * From Person Where rownum > 100 and rownum < 110;
returns 0 rows ?
Why there is no simple way to do something like Select ... FROM ... WHERE rownum BETWEEN lowerBound AND upperBound
?
How to get rid of the r
column in the resulting values? From there SQL exclude a column using SELECT * [except columnA] FROM tableA? I need apparently to create a view or a temporary table, but is there another way considering my query?
Does it ensure correct pagination? I read this article section "Pagination with ROWNUM", that said I should order the values by something unique to get a consistent pagination (so I guess ordering by rownum
is fine, if you can confirm). Doesn't it defeat the purpose of using FIRST_ROWS(N)
?
I hope it's not too much, I could split into separate questions, but I think it's relevant to have them collapsed as they are closely related.
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.