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)
Related
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.
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;
Oracle 11g
Oracle SQL Developer 3.0.03
I need to copy all of the data from a production table with +100K rows to a development table. The tables are in two different schemas.
I started to do:
select *
from prod_schema.prod_table
where ronum <= 50000 order by ID asc
Then do
select *
from prod_schema.prod_table
where ronum >= 50001
and rownum <=107161 order by ID asc
But the second query doesn't pull in any data.
Any ideas?
ROWNUM in Oracle should never be compared as greater than (>).
When you query, Oracle numbers the rows AS THEY ARE RETURNED, so you cannot even say
where rownum > 1
because as rows are discarded, they get ignored. That is, in the above case there is never a ROWNUM=1 and you get no data at all.
You can use below query while using greater than with ROWNUM in oracle:
SELECT *
FROM
(SELECT a.*, rownum rn FROM
(SELECT * FROM prod_schema.prod_table ORDER BY ID ASC) a WHERE rownum <= 107161)
WHERE rn >= 50001;
See this article: rownum only gets assigned once a row enters the result set. So where rownum > 1 will never return any row.
Do that in a single select (insert into / select * from possibly with an /*+ append */ hint, or use CTAS). Or use expdp/impdp: you can use the REMAP_SCHEMA option with that to change the schema name (and possibly restrict the selected set depending you your version of Oracle).
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.