sql 11g fetch - How do I limit the number of rows returned by an Oracle query after ordering?
You can use a subquery for this like
select *
from
( select *
from emp
order by sal desc )
where ROWNUM <= 5;
Have also a look at the topic On ROWNUM and limiting results at Oracle/AskTom for more information.
Update: To limit the result with both lower and upper bounds things get a bit more bloated with
select * from
( select a.*, ROWNUM rnum from
( <your_query_goes_here, with order by> ) a
where ROWNUM <= :MAX_ROW_TO_FETCH )
where rnum >= :MIN_ROW_TO_FETCH;
(Copied from specified AskTom-article)
Update 2: Starting with Oracle 12c (12.1) there is a syntax available to limit rows or start at offsets.
SELECT *
FROM sometable
ORDER BY name
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;
See this answer for more examples. Thanks to Krumia for the hint.
Is there a way to make an Oracle
query behave like it contains a MySQL limit
clause?
In MySQL
, I can do this:
select *
from sometable
order by name
limit 20,10
to get the 21st to the 30th rows (skip the first 20, give the next 10). The rows are selected after the order by
, so it really starts on the 20th name alphabetically.
In Oracle
, the only thing people mention is the rownum
pseudo-column, but it is evaluated before order by
, which means this:
select *
from sometable
where rownum <= 10
order by name
will return a random set of ten rows ordered by name, which is not usually what I want. It also doesn't allow for specifying an offset.
An analytic solution with only one nested query:
SELECT * FROM
(
SELECT t.*, Row_Number() OVER (ORDER BY name) MyRow FROM sometable t
)
WHERE MyRow BETWEEN 10 AND 20;
Rank()
could be substituted for Row_Number()
but might return more records than you are expecting if there are duplicate values for name.
Pagination queries with ordering are really tricky in Oracle.
Oracle provides a ROWNUM pseudocolumn that returns a number indicating the order in which the database selects the row from a table or set of joined views.
ROWNUM is a pseudocolumn that gets many people into trouble. A ROWNUM value is not permanently assigned to a row (this is a common misunderstanding). It may be confusing when a ROWNUM value is actually assigned. A ROWNUM value is assigned to a row after it passes filter predicates of the query but before query aggregation or sorting.
What is more, a ROWNUM value is incremented only after it is assigned.
This is why the followin query returns no rows:
select *
from (select *
from some_table
order by some_column)
where ROWNUM <= 4 and ROWNUM > 1;
The first row of the query result does not pass ROWNUM > 1 predicate, so ROWNUM does not increment to 2. For this reason, no ROWNUM value gets greater than 1, consequently, the query returns no rows.
Correctly defined query should look like this:
select *
from (select *, ROWNUM rnum
from (select *
from skijump_results
order by points)
where ROWNUM <= 4)
where rnum > 1;
Find out more about pagination queries in my articles on Vertabelo blog:
If you are not on Oracle 12C, you can use TOP N query like below.
SELECT *
FROM
( SELECT rownum rnum
, a.*
FROM sometable a
ORDER BY name
)
WHERE rnum BETWEEN 10 AND 20;
You can even move this from clause in with clause as follows
WITH b AS
( SELECT rownum rnum
, a.*
FROM sometable a ORDER BY name
)
SELECT * FROM b
WHERE rnum BETWEEN 10 AND 20;
Here actually we are creating a inline view and renaming rownum as rnum. You can use rnum in main query as filter criteria.
I'v started preparing for Oracle 1z0-047 exam, validated against 12c While prepping for it i came across a 12c enhancement known as 'FETCH FIRST' It enables you to fetch rows /limit rows as per your convenience. Several options are available with it
- FETCH FIRST n ROWS ONLY
- OFFSET n ROWS FETCH NEXT N1 ROWS ONLY // leave the n rows and display next N1 rows
- n % rows via FETCH FIRST N PERCENT ROWS ONLY
Example:
Select * from XYZ a
order by a.pqr
FETCH FIRST 10 ROWS ONLY
In oracle
SELECT val FROM rownum_order_test ORDER BY val DESC FETCH FIRST 5 ROWS ONLY;
VAL
10
10
9
9
8
5 rows selected.
SQL>
Same as above with corrections. Works but definitely not pretty.
WITH
base AS
(
select * -- get the table
from sometable
order by name -- in the desired order
),
twenty AS
(
select * -- get the first 30 rows
from base
where rownum <= 30
order by name -- in the desired order
)
select * -- then get rows 21 .. 30
from twenty
where rownum < 20
order by name -- in the desired order
Honestly, better to use the above answers.