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.
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
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'd like to know difference between rowID and rowNUM
And how to see both of these in our table.
when I execute this:
SELECT * FROM emp WHERE rownum=1
It returns one query but when I do the same for rowid it says
inconsistent datatypes: expected ROWID got NUMBER
And even in some of the tables, rownum returns null . Why so?
Please clarify this: rowid vs rownum?(Demo query)
Thank you
EDIT: Require to use alias to display ROWID and ROWNUM(as they're pseudocolumn)
like:
SELECT rownum r1, rowid r2 FROM emp
Both, ROWNUM and ROWID are pseudo columns.
Rowid
For each row in the database, the ROWID pseudo column returns the
address of the row.
An example query would be:
SELECT ROWID, last_name
FROM employees
WHERE department_id = 20;
More info on rowid here: https://docs.oracle.com/cd/B19306_01/server.102/b14200/pseudocolumns008.htm
Rownum
For each row returned by a query, the ROWNUM pseudo column returns 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.
You can limit the amount of results with rownum like this:
SELECT * FROM employees WHERE ROWNUM < 10;
More info on rownum here: https://docs.oracle.com/cd/B19306_01/server.102/b14200/pseudocolumns009.htm
Difference
The actual difference between rowid and rownum is, that rowid is a permanent unique identifier for that row. However, the rownum is temporary. If you change your query, the rownum number will refer to another row, the rowid won't.
So the ROWNUM is a consecutive number which applicable for a specific SQL statement only. In contrary the ROWID, which is a unique ID for a row.
Rownum (numeric) = Generated Sequence Number of your output.
Rowid (hexadecimal) = Generated automatically at the time of insertion of row.
SELECT rowid,rownum fROM EMP
ROWID ROWNUM
----- ----------------------
AAAR4AAAFAAGzg7AAA, 1
AAAR4AAAFAAGzg7AAB, 2
AAAR4AAAFAAGzg7AAC, 3
AAAR4AAAFAAGzg7AAD, 4
AAAR4AAAFAAGzg7AAE, 5
Rowid gives the address of rows or records. Rownum gives a count of records
Rowid is permanently stored in the database. Rownum is not stored in the database permanently
Rowid is automatically assigned with every inserted into a table. Rownum is a dynamic value automatically retrieved along with select statement output.
It is only for display purpose.
row id shows the unique identification for row
rownum shows the unique default series of numbers.
select * from emp
where rownum<=5; (it will execute correctly and gives output first 5 rows in your table )
select * from emp
where rowid<=5; (wrong because rowid helpful to identify the unique value)
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
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;