I want to fetch data between 10000 to 20000 rownum i have made this query
SELECT xml_to_string(XMLRECORD) FROM TABLENAME WHERE ROWNUM >10000 AND ROWNUM<=20000
but above query is not working but when i change query to
SELECT xml_to_string(XMLRECORD) FROM TABLENAME WHERE ROWNUM >0 AND ROWNUM<=20000
it works fine
what am i missing..?
Try this:
SELECT xml_to_string(XMLRECORD) FROM (select t.*, rownum rw from TABLENAME t)
WHERE rw>10000 AND rw<=20000
Rownum is calculated when Oracle retrieves the result of the query. That's why a query select * from some_table where rownum > 1 never returns anything.
In addition, without ORDER BY it doesn't make sense to get rows between 10000 and 20000. You might as well get the first 10000 (as rows are unsorted the result is unpredictable - any row can be the first).
From Oracle documentation:
For each row returned by a query, the ROWNUM pseudocolumn 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.
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.
rownum doesnot stay in any table It generates run time so when ever you run any sql statement then only for that time rownum will generate so you can't expect that you can use row num in between close.
Related
I have a stored proc in Oracle, and we're limiting the number of records with ROWNUM based on a parameter. However, we also have a requirement to know whether the search result count exceeded the arbitrary limit (even though we're only passing data up to the limit; searches can return a lot of data, and if the limit is exceeded a user may want to refine their query.)
The limit's working well, but I'm attempting to pass an OUT value as a flag to signal when the maximum results were exceeded. My idea for this was to get the count of the inner table and compare it to the count of the outer select query (with ROWNUM) but I'm not sure how I can get that into a variable. Has anyone done this before? Is there any way that I can do this without selecting everything twice?
Thank you.
EDIT: For the moment, I am actually doing two identical selects - one for the count only, selected into my variable, and one for the actual records. I then pass back the comparison of the base result count to my max limit parameter. This means two selects, which isn't ideal. Still looking for an answer here.
You can add a column to the query:
select * from (
select . . . , count(*) over () as numrows
from . . .
where . . .
) where rownum <= 1000;
And then report numrows as the size of the final result set.
You could use a nested subquery:
select id, case when max_count > 3 then 'Exceeded' else 'OK' end as flag
from (
select id, rn, max(rn) over () as max_count
from (
select id, rownum as rn
from t
)
where rownum <= 4
)
where rownum <= 3;
The inner level is your actual query (which you probably have filters and an order-by clause in really). The middle later restricts to your actual limit + 1, which still allows Oracle to optimise using a stop key, and uses an analytic count over that inner result set to see if you got a fourth record (without requiring a scan of all matching records). And the outer layer restricts to your original limit.
With a sample table with 10 rows, this gets:
ID FLAG
---------- --------
1 Exceeded
2 Exceeded
3 Exceeded
If the inner query had a filter that returned fewer rows, say:
select id, rownum as rn
from t
where id < 4
it would get:
ID FLAG
---------- --------
1 OK
2 OK
3 OK
Of course for this demo I haven't done any ordering so you would get indeterminate results. And from your description you would use your variable instead of 3, and (your variable + 1) instead of 4.
In my application I do a very simple approach. I do the normal SELECT and when the number of returned rows is equal to the limit then the client application shows LIMIT reached message, because is it very likely that my query would return more rows in case you would not limit the result.
Of course, when the number of rows is exactly the limit then this is a wrong indication. However, in my application the limit is set mainly for performance reasons by end-user, a typical limit is "1000 rows" or "10000 rows", for example.
In my case this solution is fully sufficient - and it is simple.
Update:
Are you aware of the row_limiting_clause? It was introduced in Oracle 12.1
For example this query
SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH NEXT 10 ROWS ONLY;
will return row 6 to row 16 of the entire result set. It may support you in finding a solution.
Another idea is this one:
SELECT employee_id, last_name
FROM employees
UNION ALL
SELECT NULL, NULL FROM dual
ORDER BY employee_id NULLS LAST
When you get the row where employee_id IS NULL then you know you reached the end of your result-set and no further records will arrive.
Select the whole thing, then select the count and the data, restricting the number of rows.
with
base as
(
select c1, c2, c3
from table
where condition
)
select (select count(*) from base), c1, c2, c3
from base
where rownum < 100
I'm try to select only 10 row from a table by using limit but it gives me an error,
My query is
SELECT *
FROM table_name
ORDER BY CUSTOMER
LIMIT 10
It gives an error :
ORA-00933: SQL command not properly ended
Can anyone guide me.
You can use ROWNUM :
SELECT *
FROM ( SELECT *
FROM table_name
ORDER BY CUSTOMER) t
WHERE ROWNUM <=10
For each row returned by a query, the ROWNUM pseudocolumn 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.
Or, since Oracle 12c r1, you can use FETCH :
SELECT *
FROM table_name
ORDER BY CUSTOMER
FETCH FIRST 10 ROWS ONLY
FETCH
Use this clause to specify the number of rows or percentage of rows to return. If you do not specify this clause, then all rows are returned, beginning at row offset + 1.
FIRST | NEXT
These keywords can be used interchangeably and are provided for semantic clarity.
Which of the following two SQL statements will return the desired result set (i.e. the ten rows with Status=0 and the highest StartTimes)?
Will both statements always return the same result set (StartTime is unique)?
SELECT *
FROM MyTable
WHERE Status=0
AND ROWNUM <= 10
ORDER BY StartTime DESC
SELECT *
FROM (
SELECT *
FROM MyTable
WHERE Status=0
ORDER BY StartTime DESC
)
WHERE ROWNUM <= 10
Background
My DBAdmin told me that the first statement will first limit the table to 10 rows and than order those random rows by StartTime, which is definitly not what I want.
From what I learned in that answer, the order by clause of the second statement is redundant and could be removed by an optimizer, which is also not what I want.
I asked a similar question concering the limit clause in a query to a SQLite database and am interested in understanding any differences to the above statements (using limit Vs rownum) used with an oracle db.
Your Second Query will work
Because in the first ,the first ten rows with Status 0 are selected and then the order by is done in that case the first ten rows fetched need not be in the highest order
I have the following query:
select * from abc where rownum = 10
Output: No records to display
I sure have more than 25 records in the abc table and my objective is to display the nth record.
If I write the query as: -
select * from abc where rownum = 1
it works fine and gives me the first record. Not any other record other than first.
Any idea?
Because row numbers are assigned sequentially to the rows that are fetched and returned.
Here's how that statement of yours works. It grabs the first candidate row and temporarily gives it row number 1, which doesn't match your condition so it's thrown away.
Then you get the second candidate row and it's also given row number 1 (since the previous one was tossed away). It doesn't match either.
Then the third candidate row ... well, I'm sure you can see where this is going now. In short, you will never find a row that satisfies that condition.
Row numbers are only useful for = 1, < something or <= something.
This is all explained in the Oracle docs for the rownum pseudo-column.
You should also keep in mind that SQL is a relational algebra that returns unordered sets unless you specify an order. That means row number ten may be something now and something else in three minutes.
If you want a (kludgy, admittedly) way to get the nth row, you can use something like (for the fifth row):
select * from (
select * from (
select col1, col2, col3 from tbl order by col1 asc
) where rownum < 6 order by col1 desc
) where rownum = 1
The inner select will ensure you have a consistent order on the query before you start throwing away rows, and the middle select will throw away all but the first five rows from that, and also reverse the order.
The outer select will then only return the first row of the reversed set (which is the last row of the five-row set when it was in ascending order).
A better way is probably:
select * from (
select rownum rn, col1, col2, col3 from tbl order by col1
) where rn = 5
This works by retrieving everything and assigning the rownum to a "real" column, then using that real column number to filter the results.
I have a query in oracle to fetch data from table using rownum but i didn't get any data.
My query is like this :
select * from table-name where rownum<5
is this is a wrong query to fetch data whose row number is less than 5.
when i used query like :
select * from table-name where rownum<=4
than it will gives a result record.
My question is what is wrong here with ?
Is this is syntax error or anything else??..
rownum is a pseudo column that counts rows in the result set after the where clause has been applied.
SELECT table_name
FROM user_tables
WHERE rownum > 2;
TABLE_NAME
------------------------------
0 rows selected
However, this query will always return zero rows, regardless of the number of rows in the table.
To explain this behaviour, we need to understand how Oracle processes ROWNUM. When assigning ROWNUM to a row, Oracle starts at 1 and only increments the value when a row is selected; that is, when all conditions in the WHERE clause are met. Since our condition requires that ROWNUM is greater than 2, no rows are selected and ROWNUM is never incremented beyond 1.
http://blog.lishman.com/2008/03/rownum.html
another stackoverflow link
Edited
this paragraph i find on oracle website which is much better
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:
Syntax seems correct to me.
However ROWNUM is calculated on result rows for example:
SELECT * FROM TABLE_NAME WHERE ROWNUM < 10 ORDER BY TABLE_FIELD ASC;
and
SELECT * FROM TABLE_NAME WHERE ROWNUM < 10 ORDER BY TABLE_FIELD DESC;
Will give you different results.
Each time a query is executed, for each tuple, Oracle assigns a ROWNUM which is scopet to that only query.
What are you trying to accomplish ?
Since for the reasons rahularyansharma mentions, rownum based queries won't always function the way you might expect, a way around this is to do something like
SELECT * from (SELECT rownum AS rn, TABLE_NAME.* FROM TABLE_NAME)
where rn > 5;
However, be aware that this will be a fairly inefficient operation when operating over large datasets.