Oracle Fetch Next Row Limit by unique id's - sql

Question

I have a table having data as shown below,
+-------+----------------+----------------+
| Id | HierUnitId | ObjectNumber |
+-------+----------------+----------------+
| 10 | 3599 | 1 |
| 10 | 3599 | 2 |
| 20 | 3599 | 3 |
| 20 | 3599 | 4 |
| 20 | 3599 | 1 |
| 30 | 3599 | 2 |
| 30 | 3599 | 3 |
+-------+----------------+----------------+
I have a select query
SELECT ID FROM TEST
FETCH NEXT :LIMIT ROWS ONLY
Now I want to limit the number of rows using the value of limit.
When the value of Limit is 2 I want two distinct id's i.e up to 5 rows. However, from query I will get only two rows having 10 as the id. Can someone help me in limiting the rows using distinct id?
What i want is total number of distinct id in the output is limit.

Answer 1

Use the DENSE_RANK analytic function to number the rows based on the unique/distinct ID values and then filter on that:
SELECT id
FROM (
SELECT ID,
DENSE_RANK() OVER (ORDER BY id) AS rnk
FROM test
)
WHERE rnk <= 2;
Which, for the sample data:
CREATE TABLE test (Id, HierUnitId, ObjectNumber ) AS
SELECT 10, 3599, 1 FROM DUAL UNION ALL
SELECT 10, 3599, 2 FROM DUAL UNION ALL
SELECT 20, 3599, 3 FROM DUAL UNION ALL
SELECT 20, 3599, 4 FROM DUAL UNION ALL
SELECT 20, 3599, 1 FROM DUAL UNION ALL
SELECT 30, 3599, 2 FROM DUAL UNION ALL
SELECT 30, 3599, 3 FROM DUAL;
Outputs:
ID
10
10
20
20
20
db<>fiddle here

Answer 2

As you said in the comment, you need to be able to define how many distinct ids should be shown. For that case i'd recommend you to find those ids first (see the distinct_ids part) and fetch all the lines you needed afterwards
with distinct_ids as (
select distinct id
from test_data
order by id
fetch first :limit rows only)
select id
from test_data td
join distinct_ids di
on td.id = di.id

Answer 3

If you need some distinct IDs without any particular order, then you may put fetch next ... into the subquery with distinct keyword. Index on ID column will be suitable to avoid two full table scans (I assume that ID cannot be null)
select /*+gather_plan_statistics*/
*
from t
where id in (
select distinct id
from t
where id is not null
fetch next 2 rows only
)
ID | HIERUNITID | OBJECTNUMBER
-: | ---------: | -----------:
1 | 3599 | 1
1 | 3599 | 2
2 | 3599 | 3
2 | 3599 | 4
2 | 3599 | 1
select *
from table(dbms_xplan.display_cursor(
format => 'ALL -PROJECTION -ALIAS ALLSTATS LAST'
))
| PLAN_TABLE_OUTPUT |
| :------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| SQL_ID 2sqqq53kpy5rj, child number 0 |
| ------------------------------------- |
| select /*+gather_plan_statistics*/ * from t where id in ( select |
| distinct id from t where id is not null fetch next 2 rows only ) |
| |
| Plan hash value: 534568331 |
| |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem | |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| | 0 | SELECT STATEMENT | | 1 | | | 5 (100)| | 5 |00:00:00.01 | 3 | | | | |
| | 1 | MERGE JOIN SEMI | | 1 | 5 | 115 | 5 (40)| 00:00:01 | 5 |00:00:00.01 | 3 | | | | |
| | 2 | TABLE ACCESS BY INDEX ROWID| T | 1 | 7 | 70 | 2 (0)| 00:00:01 | 6 |00:00:00.01 | 2 | | | | |
| | 3 | INDEX FULL SCAN | T_IX | 1 | 7 | | 1 (0)| 00:00:01 | 6 |00:00:00.01 | 1 | | | | |
| |* 4 | SORT UNIQUE | | 6 | 2 | 26 | 3 (67)| 00:00:01 | 5 |00:00:00.01 | 1 | 2048 | 2048 | 2048 (0)| |
| | 5 | VIEW | VW_NSO_1 | 1 | 2 | 26 | 2 (50)| 00:00:01 | 2 |00:00:00.01 | 1 | | | | |
| |* 6 | VIEW | | 1 | 2 | 20 | 2 (50)| 00:00:01 | 2 |00:00:00.01 | 1 | | | | |
| |* 7 | WINDOW NOSORT STOPKEY | | 1 | 3 | 39 | 2 (50)| 00:00:01 | 2 |00:00:00.01 | 1 | 73728 | 73728 | | |
| | 8 | VIEW | | 1 | 3 | 39 | 2 (50)| 00:00:01 | 2 |00:00:00.01 | 1 | | | | |
| | 9 | SORT UNIQUE NOSORT | | 1 | 3 | 9 | 2 (50)| 00:00:01 | 2 |00:00:00.01 | 1 | | | | |
| |* 10 | INDEX FULL SCAN | T_IX | 1 | 7 | 21 | 1 (0)| 00:00:01 | 6 |00:00:00.01 | 1 | | | | |
| -------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| |
| Predicate Information (identified by operation id): |
| --------------------------------------------------- |
| |
| 4 - access("ID"="ID") |
| filter("ID"="ID") |
| 6 - filter("from$_subquery$_004"."rowlimit_$$_rownumber"<=2) |
| 7 - filter(ROW_NUMBER() OVER ( ORDER BY NULL)<=2) |
| 10 - filter("ID" IS NOT NULL) |
| |
db<>fiddle here