This question already has answers here:
using of rownum function with ">" sign in oracle [duplicate]
(3 answers)
How do I limit the number of rows returned by an Oracle query after ordering?
(14 answers)
how to select even records from a table in oracle?
(12 answers)
Oracle "LIMIT n,m" equivalent [duplicate]
(1 answer)
Closed 3 years ago.
I've created a table that has 1.8m rows and, while I can extract to CSV, it cannot be opened. This is not a query that is ran often and the end users have very low technical ability. Therefore I wanted a quick and dirty fix that had little time investment.
My solution was to split the query into chunks:
SELECT *
FROM my_table
WHERE ROWNUM BETWEEN 0 AND 500000
The above works fine however,
SELECT *
FROM my_table
WHERE ROWNUM BETWEEN 500001 AND 1000000
the next iteration does NOT.
I've checked that there are definitely 1.8m rows.
What's going on here? What am I missing?
Rownum is applied on query result, not on the table. Apply rownum to get first 1m rows and then filter out the first 0.5m.
select *
from (
SELECT t.*, rownum rn
FROM my_table t
WHERE ROWNUM BETWEEN 0 AND 1000000
) where rn > 500000
Also, as pointed out by #Connor in comments, rownum filter by itself may not produce deterministic results. Use an order by clause to apply appropriate ordering. In that case, the SQL changes to:
select *
from (
SELECT t.*, rownum rn
FROM (
select *
from my_table
order by x, y, z -- apply appropriate sort order here
) t
WHERE ROWNUM BETWEEN 0 AND 1000000
) where rn > 500000
This is becomes tedious to write. In Oracle 12c+, the new FETCH FIRST/NEXT syntax is much more concise:
select *
from my_table
order by x, y, z -- apply appropriate sort order here
offset 500000 rows
fetch next 500000 rows only;
Related
This question already has answers here:
How to get the top 10 values in postgresql?
(4 answers)
Closed 7 months ago.
i am using postgresql. i want to be able to select or delete the first n rows after sorting the ascendingly according to the timestamp of insertion.
i found an example but it uses a table named logtable, i do not have that logtable.
in otherwords, i want to sort the records in the table based on the time of data insertion into the table then selected the first N rows
please let me know how to achieve this task
You could use ROW_NUMBER:
WITH cte AS (
SELECT *, ROW_NUMBER() OVER (ORDER BY ts_insertion) rn
FROM yourTable
)
SELECT *
FROM cte
WHERE rn <= N -- replace N with your actual limit value
ORDER BY ts_insertion;
If you only want the first n rows, you can use LIMIT n, a window function is not necessary.
You can also use OFFSET:
SELECT column1, column2
FROM table_name
ORDER BY column1 ASC
LIMIT row_count OFFSET row_to_skip;
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
This question already has answers here:
How ROWNUM works in pagination query?
(3 answers)
Closed 7 years ago.
I observed a strange behaviour with ROWNUM in Oracle with static tables. I am trying pagination with query like below
select * from (
select the_data,rownum as seqn from the_table
) where seqn <= somenumber and seqn >= othernumber
However when I execute query multiple times I observe that the rownum value returned is not unique for a row ie if a row appears at rownum 25 for the first time it appears at rownum 125 for second execution
This issue does not appear if I do a order by like,
select * from (
select the_data,rownum as seqn from the_table order by column3
) where seqn <= somenumber and seqn >= othernumber
However order by decreases performance. Is this normal or is there any other way to improve performance.
You should check oracle documents for this question. Followed links may be helpful for you http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions156.htm#SQLRF06100
One example:
SELECT * FROM ( SELECT ROW_NUMBER() OVER () AS R, T.* FROM T ) AS TR WHERE R <= 10;
T is a table name. R is a row
This question already has answers here:
How do I limit the number of rows returned by an Oracle query after ordering?
(14 answers)
Closed 2 years ago.
I want do sorting by property ALL data in my db and ONLY AFTER that use LIMIT and OFFSET.
Query like this:
SELECT select_list
FROM table_expression
[ ORDER BY ... ]
[ LIMIT { number | ALL } ] [ OFFSET number ]
I know the sorting ends as soon as it has found the first row_count rows of the sorted result. Can I do sorting all data before calling LIMIT and OFFSET?
Prior to 12.1, Oracle does not support the LIMIT or OFFSET keywords. If you want to retrieve rows N through M of a result set, you'd need something like:
SELECT a.*
FROM (SELECT b.*,
rownum b_rownum
FROM (SELECT c.*
FROM some_table c
ORDER BY some_column) b
WHERE rownum <= <<upper limit>>) a
WHERE b_rownum >= <<lower limit>>
or using analytic functions:
SELECT a.*
FROM (SELECT b.*,
rank() over (order by some_column) rnk
FROM some_table)
WHERE rnk BETWEEN <<lower limit>> AND <<upper limit>>
ORDER BY some_column
Either of these approaches will sort give you rows N through M of the sorted result.
In 12.1 and later, you can use the OFFSET and/or FETCH [FIRST | NEXT] operators:
SELECT *
FROM some_table
ORDER BY some_column
OFFSET <<lower limit>> ROWS
FETCH NEXT <<page size>> ROWS ONLY