I observed that rownum can be used as follows:
1) select * from emp where rownum<5;
2) select * from emp where rownum<=5;
3) select * from emp where rownum=1;
All this queries return expected output. Say table emp has 7 rows then 1st query returns 4 rows, 2nd returns 5 rows ,3rd returns 1 row.
But when I try to use something like:
4) select * from emp where rownum=5;
5) select * from emp where rownum>5;
6) select * from emp where rownum>=5;
7) select * from emp where rownum between 5 and 10;
In all this cases, it returns 0 rows. Why this is so? Is there any specific reason for this?
This is because rownum is a pseudo column and it counts rows in the result set after you apply the where clause.
From the Oracle docs:-
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;
The first row fetched is assigned a ROWNUM of 1 and makes the condition false. The second row to be fetched is now the first row and is also assigned a ROWNUM of 1 and makes the condition false. All rows subsequently fail to satisfy the condition, so no rows are returned.
You can also use ROWNUM to assign unique values to each row of a table, as in this example:
UPDATE my_table
SET column1 = ROWNUM;
You may find these references helpfull:-
Using ROWNUM in Oracle
SQL Query With Row_Number, order by and where clause
You can do something similar like this:
select * from (
select
emp.*,
row_number() over (order by fieldname) as rnum
from emp
) tmp
where
rnum between 5 and 10;
Related
I am writing a very simple query for an Oracle DB (version 9).
Somehow I can get first 5 rows:
select * from cities where rownum <= 5
But skipping 5 rows returns an empty result:
select * from cities where rownum >= 5
Using:
Oracle SQL Developer
Oracle DB version 9
Why is the second query returning an empty result?
In Oracle Database 12c (release 1) and above, you can do this very simple, for skip 5 rows:
SELECT * FROM T OFFSET 5 ROWS
and for skip 5 rows and take 15 rows:
SELECT * FROM T OFFSET 5 ROWS FETCH NEXT 15 ROWS ONLY
You can use the following query to skip the first not n of rows.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts
) where rec_no > <<startRowNum - n>>
The above query is similar to pagination query below.
select * from (
select rslts.*, rownum as rec_no from (
<<Query with proper order by (If you don't have proper order by you will see weird results)>>
) rslts where rownum <= <<endRowNum>>
) where rec_no > <<startRowNum>>
Your cities query:
select * from (
select rslts.*, rownum as rec_no from (
select * from cities order by 1
) rslts
) where rec_no > 5 <<startRowNum>>
Note: Assume first column in cities table is unique key
Oracle increments rownum each time it adds a row to the result set. So saying rownum < 5 is fine; as it adds each of the first 5 rows it increments rownum, but then once ruwnum = 5 the WHERE clause stops matching, no more rows are added to the result, and though you don't notice this rownum stops incrementing.
But if you say WHERE rownum > 5 then right off the bat, the WHERE clause doesn't match; and since, say, the first row isn't added to the result set, rownum isn't incremented... so rownum can never reach a value greater than 5 and the WHERE clause can never match.
To get the result you want, you can use row_number() over() in a subquery, like
select *
from (select row_number() over() rn, -- other values
from table
where -- ...)
where rn > 5
Update - As noted by others, this kind of query only makes sense if you can
control the order of the row numbering, so you should really use row_number() over(order bysomething) where something is a useful ordering key in deciding which records are "the first 5 records".
rownum is being increased only when a row is being output, so this type of condition won't work.
In any case, you are not ordering your rows, so what's the point?
Used row_number() over (order by id):
select * from
(select row_number() over (order by id) rn, c.* from countries c)
where rn > 5
Used ROWNUM:
select * from
(select rownum rn, c.* from countries c)
where rn > 5
Important note:
Using alias as countries c instead of countries is required! Without, it gives an error "missing expression"
Even better would be:
select * from mytab sample(5) fetch next 1 rows only;
Sample clause indicates the probability of each row getting picked up in the sampling process. FETCH NEXT clause indicates the number of rows you want to select.
With this code, you can query your table with skip and take.
select * from (
select a.*, rownum rnum from (
select * from cities
) a
) WHERE rnum >= :skip + 1 AND rnum <= :skip + :take
This code works with Oracle 11g. With Oracle 12, there is already a better way to perform this queries with offset and fetch
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.
I have to select n number of rows from an oracle database from sap using native sql. In open sql the query would be like
select * from myDB where size > 2000 upto n rows.
what I learnt from other posts the equivalent native sql query would be
SELECT * FROM myDB WHERE SIZE > 2000 AND rownum <= 100
is this correct?
Do I need to have rownum as one of the fields in the DB table?
SELECT *
FROM myDB
WHERE SIZE > 2000
AND ROWNUM <= 100
Is a syntactically correct query.
Do I need to have rownum as one of the fields in the DB table?
No, the ROWNUM pseudocolumn assigns, for each row returned by a query, a number indicating the order in which Oracle selects the row from a table or set of joined rows. The first row selected has a ROWNUM of 1, the second has 2, and so on.
Note that ROWNUM is applied before any ORDER BY clauses.
So:
SELECT *
FROM myDB
WHERE ROWNUM <= 10
ORDER BY column_name
Will select 10 rows (it could be any 10 rows and not necessarily the 10 rows which are first in the desired ordering) and then will order those 10 rows by the desired column.
If you want to get the rows with the top-N values for a column then you need to apply the ORDER BY first and then restrict the number of rows:
SELECT *
FROM ( SELECT *
FROM myDB
ORDER BY column_name )
WHERE ROWNUM <= 10
If you are using Oracle 12 then they have implemented a new syntax for TOP-N queries:
SELECT *
FROM myDB
ORDER BY column_name
FETCH FIRST 10 ROWS ONLY;
ROWNUM is a pseudocolumn generated by Oracle whenever you perform a SELECT. The assignment of the value to ROWNUM is the last thing done before the query returns a row - thus, the first row emitted by the query is given ROWNUM = 1, the second row emitted by the query is given ROWNUM = 2, and so on. Notice: this means that a statement like the following WILL RETURN NO ROWS:
SELECT *
FROM SOME_TABLE
WHERE ROWNUM >= 2
Why does this return no rows? It's because the first row which would be emitted by the query is always given ROWNUM = 1, and since the query is looking only for rows with ROWNUM >= 2 no rows will be selected because the first ROWNUM value of 1 is applied to the first row emitted.
HOWEVER - if you really want to get all rows EXCEPT the first (or the first 10, or what have you) you can do it like this:
SELECT *
FROM (SELECT *, ROWNUM AS INNER_ROWNUM
FROM SOME_TABLE
WHERE SOMETHING = SOMETHING_ELSE
ORDER BY WHATEVER) i
WHERE i.INNER_ROWNUM IN (1, 2, 3, 5, 7, 11, 13, 17, 19);
Here we use an inner SELECT which gets ALL rows for the given condition (SOMETHING = SOMETHING_ELSE) and assigns the ROWNUM from the INNER query to a computed column named INNER_ROWNUM, which we can then use as a normal column in the outer query.
Use rownum concept it is a pseudo column in oracle by using this concept we can achieve the TOP keyword functionality.
select col1,col2,col3..coln from
(
select rownum rn,e.* from mydb where size>2000
)
where rn>N
Note: Col1 to coln are column names of your table.
write a SQL query to retrieve third row from the table ,Let 's say table name is emp
Actually i want to understand rownum concept
Select * from
(select rownum r ,e.* from emp e )
where r = 3 ;
and it works well
Once i try this
select * from
( select rownum , e.* from emp e )
where rownum = 3 ;
i'm getting my output : no data found
could you explain please , Thanks in advance
First of all no need of including rownum in your inner query. From Oracle documentation for ROWNUM a small excerpt
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;
The first row fetched is assigned a ROWNUM of 1 and makes the
condition false. The second row to be fetched is now the first row and
is also assigned a ROWNUM of 1 and makes the condition false. All rows
subsequently fail to satisfy the condition, so no rows are returned.
I believe, that's the same thing happening for your case as well with the condition WHERE ROWNUM = 3.
The first row fetched is assigned a ROWNUM of 1 and makes the condition false cause 1 != 3. The second row to be fetched is now the first row and is also assigned a ROWNUM of 1 and makes the condition false cause again 1 != 3 and so on ...
It's recommended to use ROW_NUMBER instead for this purpose.
Actually i want to understand rownum concept
Each query has its own rownum. ROWNUM is not a real column that propagates from the inside query to the outside. ROWNUM is a pseudocolumn for each query and is valid within that query scope only. The inner rownum is rownum.1, the outer rownum is rownum.2, they are not the same, so you are dealing with 2 separate columns. I you had 3 queries, nested, you would have 3 rownums. The outer ROWNUM shadows the inner ROWNUM. To pass ROWNUM from an inner subquery to an outer query requires you to alias the column to something besides a pseudocolumn, otherwise the column is "redefined" with each outer query.
Your query could be thought of as:
select rownum, subquery.sub_rownum, subquery.empno from
( select rownum as sub_rownum, e.* from emp e ) subquery
where subquery.sub_rownum = 3
;
Notice there are now 2 "rownum" columns. The inner rownum, by its alias, sub_rownum, appears to be a real column to the outer queries. Notice the use of rownum again in the outer query. They are 2 different variables. Note the values 1 and 3. ROWNUM = 1 on the outside because its the 1st row in the outer result set. SUB_ROWNUM = 3 because it was the 3rd result in the inner result set, and since you aliased it, it survived to bubble up as a real column.
ROWNUM SUB_ROWNUM EMPNO
---------- ---------- ----------
1 3 7521
For SQL Server:
SELECT * FROM
(
SELECT
ROW_NUMBER() OVER (ORDER BY id) AS RowNumber, e.*
FROM dbo.tbl_emp e
)demo
WHERE demo.RowNumber=3
How to get random output of multiple rows from any table in Oracle? Also what is the meaning of (dbms_random.value)
select *
from (select *
from emp
order by dbms_random.value)
where rownum=1;
It gives only one random output. How do I get multiple?
try
select * from (select * from emp order by dbms_random.value) where rownum<10
10 can be replaced by any other count of your choice