I have a query
select * from tablename where rownum =1;
This query is giving desired result but if i run
select * from tablename where rownum=2;
Then this query is giving null result.
Any idea why?
The predicate WHERE rownum=2 can never be true because of the way ROWNUM is assigned. If there are 5 rows in the table, for example, the algorithm would go something like
Retrieve the first row from the table. Assign it a rownum of 1.
Check this rownum against the predicate, discard
Retrieve the second row from the table. Assign it a rownum of 1.
Check this rownum against the predicate, discard
Retrieve the third row from the table. Assign it a rownum of 1.
Check this rownum against the predicate, discard
Retrieve the fourth row from the table. Assign it a rownum of 1.
Check this rownum against the predicate, discard
Retrieve the fifth row from the table. Assign it a rownum of 1.
Check this rownum against the predicate, discard
Since there is no "first row" in the result set, there can be no "second row". You can use ROWNUM in inequality comparisons, i.e.
SELECT *
FROM table_name
WHERE rownum <= 2
will return 2 rows.
If you are trying to identify the "second row" in a table, you'd likely want something like
SELECT *
FROM (SELECT t.*,
rank() over (order by some_column) rnk
FROM table_name)
WHERE rnk = 2
I don't think this is a well defined query:
ROWNUM is a pseudo-column that returns a row's position in a result
set. ROWNUM is evaluated AFTER records
are selected from the database and
BEFORE the execution of ORDER BY
clause.
Related
This question already has answers here:
How do I limit the number of rows returned by an Oracle query after ordering?
(14 answers)
Closed 5 years ago.
I want to select the number of rows which are greater than 3 by rownum function i_e "(rownum>3)"
for example if there are 25 rows and I want to retrieve the last 22 rows by rownum function.
but when I write the
select * from test_table where rownum>3;
it retrieve no row.
can any one help me to solve this problem.
thanks in advance
In RDBMS there is no first or last rows. What you calls "raws" , actually is set(sets), they can be ordered or not. rownum is a function, which is just enumerates result set, it makes sense only after set is calculated, to order your set of data (rows) you should do it in your query before rownum call, you must tell DB what means for the order in particular select statement.
It is not working because: for the first row assumes the ROWNUM of 1 and since your WHERE clause is ROWNUM>3 then this reduces to 1>3 and the row is discarded. The subsequent row will then be tested against a ROWNUM of 1 (since the previous row is no longer in the output and now does not require a row number), which will again fail the test and be discarded. Repeat, ad nauseum and all rows fail the WHERE clause filter and are discarded.
If you want to assign the rows a ROWNUM then you need to do this is a sub-query:
SELECT * -- Finally, in the outer query, filter on the assigned ROWNUM
FROM (
SELECT t.*, -- First, in the inner sub-query, apply a ROWNUM
ROWNUM AS rn
FROM test_table t
)
WHERE rn > 3;
Or, if you want to order the results before numbering:
SELECT * -- Finally, in the outer query, filter on the assigned ROWNUM
FROM (
SELECT t.*, -- Second, in the next level sub-query, apply a ROWNUM
ROWNUM AS rn
FROM (
SELECT * -- First, in the inner-most sub-query, apply an order
FROM test_table
ORDER BY some_column
) t
)
WHERE rn > 3;
select * from (select rownum as rn, t.* from test_table t) where rn > 3
see this article for more samples
On Top-n and Pagination Queries By Tom Kyte
Given;
CREATE TABLE T1 (ID INTEGER, DESCRIPTION VARCHAR2(20));
INSERT INTO T1 VALUES (1,'ONE');
INSERT INTO T1 VALUES (2,'TWO');
INSERT INTO T1 VALUES (3,'THREE');
INSERT INTO T1 VALUES (4,'FOUR');
INSERT INTO T1 VALUES (5,'FIVE');
COMMIT;
Why does;
SELECT * FROM
( SELECT ROWNUM, ID, DESCRIPTION
FROM T1)
WHERE MOD(ROWNUM,1)=0;
Return
ROWNUM ID DESCRIPTION
------ -------------------------------------- --------------------
1 1 ONE
2 2 TWO
3 3 THREE
4 4 FOUR
5 5 FIVE
Whereas;
SELECT * FROM
( SELECT ROWNUM, ID, DESCRIPTION
FROM T1)
WHERE MOD(ROWNUM,2)=0;
Return zero rows ???
Confused, expected ROWNUM=(2,4) to be returned...
SELECT B.* FROM
( SELECT ROWNUM a, ID, DESCRIPTION
FROM T1) B
WHERE MOD(A,2)=0;
Reason: Your approach involves running rownum twice. You don't need to; nor really do you want to. Based on order of operations, the where clause will execute before the the outer select; which means the select hasn't determined the values for each row, and the number of rows is not known yet.
Additional:
I would recommend adding an order by to the inline view so the rownumbers are in a expected specific order as opposed to what the engine derives.
You have 2 operations of ROWNUM.
The 1st ROWNUM generates the numbers 1 through 5.
The 2nd ROWNUM doesn't generate anything because for the row the ROWNUM value is 1, but since MOD(1,2)=0 is false, the record is not being outputted and the ROWNUM is not being incremented, failing the condition again and again.
This query, using alias, returns exactly what you have expected:
SELECT * FROM
( SELECT ROWNUM as rn, ID, DESCRIPTION
FROM T1)
WHERE MOD(rn,2)=0;
Some facts about the ROWNUM pseudo column in Oracle:
The ROWNUM assigned to each row is determined by the order in which Oracle retrieves the row from the DB.
The order in which rows are returned is non deterministic, such that running it once may return rows in one ordering, and a second time around may have a different ordering if the base tables have been reorganized, or Oracle uses a different query plan.
The order in which ROWNUMs are assigned to rows is not necessarily correlated with the that of an order by clause (the order by clause may affect the ROWNUM order since it may cause a different query plan to be used, but the ROWNUMbers are unlikely to match the sort order).
ROWNUMbers are assigned after the records are filtered by the WHERE clause, so if you filter out ROWNUM 1 you will never return any records.
Filtering a subquery that returns an aliased ROWNUM column works because the entire subquery is returned to the outer query before the outer query filters the rows, but the ROWNUMs will still have a non deterministic order.
To successfully return a top N or Nth row query in a deterministic fashion you need to assign row numbers in a deterministic way. One such way is to use the the `ROW_NUMBER' analytic function in a subquery:
select * from
(select ROW_NUMBER() over (order by ID) rn
, ID
, DESCRIPTION
from T1)
where rn <= 4 -- Top N
or
where rn = 4 -- 1st Nth row
or even
WHERE MOD(rn,2)=0 -- every Nth row
In either case the ORDER BY clause in the ROW_NUMBER analytic function needs to match the granularity of the data otherwise ties in the ordering will again be non deterministic, most likely matching the current ROWNUM ordering.
I have some values as below:
MYSTRING
--------
Dharshan
Ramalingam
Devid
I can select the specific row value through below query
select * from ( select mystring , rownum rn FROM teststring )
where rn = 2;
tell the way to delete this second row giving the row no and give me the brief explanation . I have tried as below but its not work....
delete from testring where rownum=2;
select * from ( select mystring , rownum rn FROM teststring ) where rn = 2;
Your query returns rows randomly without a specified order. A heap-organized table is a table with rows stored in no particular order. This is a standard Oracle table.
From Oracle documentation,
Without an order_by_clause, no guarantee exists that the same query executed more than once will retrieve rows in the same order.
delete from testring where rownum=2;
Your above query shows you do not understand how ROWNUM works.
Your query will never delete any rows.
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 = 2;
Because ROWNUM = 2 is not true for the first row, ROWNUM does not advance to 2. Hence, no ROWNUM value ever gets to be greater than 1.
How to correctly use ROWNUM:
As you wanted to select a row based on ROWNUM, you could do something like pagination:
SQL> SELECT empno
2 FROM
3 (SELECT empno, sal, ROWNUM AS rnum FROM
4 ( SELECT empno, sal FROM emp ORDER BY sal
5 )
6 )
7 WHERE rnum =2;
EMPNO
----------
7900
It happens in three levels:
Innermost sub-query first sorts the rows based on the ORDER BY clause.
In second level, the sub-query assigns ROWNUM
The outermost query filters the rows based on the ROWNUM given by inner query which is no more a pseudo-column but the sub-query resultset.
if you want to delete all rows except one(just for understanding how the rownum works):
delete
from table
where rowid in (select rwid
from (select rownum as rn, rwid
from( select rowid as rwid from table)
order by rwid)
where rn > 1);
or, more simpler:
delete from table
where rowid <> (select rowid from table where rownum = 1);
Rownum is a pseudocolumn.
The rownum is not assigned permanently to the row. It means you can't build your query based on this criteria.
You should instead delete your row with
delete from teststring where mystring = 'RAMALINGAM';
I have tried this Query and it's working fine.
DELETE FROM NG_USR_0_CLIENT_GRID_NEW WHERE rowid IN
( SELECT rowid FROM
(
SELECT wi_name, relationship, ROW_NUMBER() OVER (ORDER BY rowid DESC) RN
FROM NG_USR_0_CLIENT_GRID_NEW
WHERE wi_name = 'NB-0000001385-Process'
)
WHERE RN=2
);
Please suggest that can we improve it's performance.
Any comment is appreciated.
This question already has answers here:
How ROWNUM works in pagination query?
(3 answers)
Closed 3 years ago.
When I am using rownum and between keywords then the query doesn't return any rows. Can anyone explain the reason why query is not retrieving any rows?
select * from cus where rownum between 2 and 6;
I just want to check whether rownum will work when it is used with between keyword .So ,I just tried the above query to display the rows which are in between 2 and 6. But when I tried to execute the query, it doesn't retrieve any rows.
thanks in advance
Oracle rownum starts at 1, so you will never get the first rownum if you say between 2 and N.
It takes a row to "initiate" the rownum pseudocolumn sequence, so by eliminating rownum 1 in your criteria, you eliminate all rownums (or every row essentially has rownum 0).
Look at it like this. You don't get a ROWNUM until the database returns a row to you. The first row of any criteria will always be ROWNUM 1.
Now, the trick you can use is to use a subquery. Each subquery will have its own rownum, and if you alias it to another column name, you can preserve it into outer queries, and treat it however you like. So if you are looking to implement paging of a result set, you would normally alias rownum from inner results as rownum_ to an outer subquery to limit with BETWEEN.
select * from
(select t.*, rownum as rownum_ from t)
where rownum_ between 2 and 6
But note, that the outer result set will have its own rownum, so you could do:
select t2.*, rownum from
(select a, b, rownum as rownum_ from t) t2
where rownum_ between 2 and 6
You will see rownum on the final result still starts at 1, but your inner result will have rownum_ starting at 2.
select * from cus where rownum between 2 and 6;
That is completely wrong. Because, ROWNUM is a pseudo-column which increments to 2 only when it started at ROW one(random, of course). In your query, the predicate ROWNUM BETWEEN 2 AND 6 is meaningless, since ROWNUM has not yet been assigned.
Let's understand this step-by-step :
Understand how a SQL statement is interpreted. After the query is parsed, the filter is applied.
In your query, the filter ROWNUM BETWEEN 2 AND 6 is meaningless, since, Oracle has not yet assigned ROWNUM 1 as the first row is
not yet fetched.
When the first row is fetched, then ROWNUM is assigned as a pseudo-number. But the filter in your query directly points to rows
between 2 and 6, which is absurd. So, no rows are returned.
Mysql doesnt have rownum.
If you are looking for oracle maybe you can try something like this:
select * from (select cus.*, rownum as row_num from cus)
where row_num between 2 and 6;
or
select * from (select cus.*, rownum as row_num from cus)
where row_num >1 and row_num <=6;
SELECT id
FROM (
SELECT id
FROM table
WHERE
PROCS_DT is null
ORDER BY prty desc, cret_dt ) where rownum >0 and rownum <=100
The above query is giving me back 100 records as expected
SELECT id
FROM (
SELECT id
FROM table
WHERE
PROCS_DT is null
ORDER BY prty desc, cret_dt ) where rownum >101 and rownum <=200
why is the above query returning me zero records?
Can some one help me how i can keep on. I am dumb in oracle...
Try this:
SELECT id
FROM
(SELECT id,
rownum AS rn
FROM
(SELECT id
FROM TABLE
WHERE PROCS_DT IS NULL
ORDER BY prty DESC, cret_dt) )
WHERE rn >101
AND rn <=200
If you are comfortable using the ANALYTIC functions, try this:
SELECT id
FROM
(
SELECT id,
ROW_NUMBER() OVER(ORDER BY prty DESC, cret_dt ) rn
FROM table
WHERE procs_dt IS NULL
)
WHERE rn >101 and rn <=200
ROWNUM values are assigned to rows as they are returned from a query (or subquery). If a row is not returned, it is not assigned a ROWNUM value at all; so the ROWNUM values returned always begin at 1 and increment by 1 for each row.
(Note that these values are assigned prior to any sorting indicated by the ORDER BY clause. This is why in your case you need to check rownum outside the subquery.)
The odd bit of logic you have to understand is that when you have a predicate on ROWNUM, you are filtering on a value that will only exist if the row passes the filter. Conceptually, Oracle applies any other filters in the query first, then tentatively assigns ROWNUM 1 to the first matching row and checks it against the filter on ROWNUM. If it passes this check, it will be returned with that ROWNUM value, and the next row will be tentatively assigned ROWNUM 2. But if it does not pass the check, the row is discarded, and the same ROWNUM value is tentatively assigned to the next row.
Therefore, if the filter on ROWNUM does not accept a value of 1, no rows will ever pass the filter.
The use of the analytic function ROW_NUMBER() shown in the other answers is one way around this. This function explicitly assigns row numbers (distinct from ROWNUM) based on a given ordering. However, this can change performance significantly, as the optimizer does not necessarily realize that it can avoid assigning numbers to ever possible row in order to complete the query.
The traditional ROWNUM-based way of doing what you want is:
SELECT id
FROM (
SELECT rownum rn, id
FROM (
SELECT id
FROM table
WHERE
PROCS_DT is null
ORDER BY prty desc, cret_dt
) where rownum <=200
) where rn > 101
The innermost query conceptually finds all matching rows and sorts them. The next layer assigns ROWNUMs to these and returns only the first 200 matches. (And actually, the Oracle optimizer understands the significance of a sort followed by a ROWNUM filter, and will usually do the sort in such a way as to identify the top 200 rows without caring about the specific ordering of the other rows.)
The middle layer also takes the ROWNUMs that it assigns and returns them as part of its result set with the alias "rn". This allows the outermost layer to filter on that value to establish the lower limit.
I would experiment with this variant and the analytic function to see which performs better in your case.