In Oracle, how do I get a page of distinct values from sorted results? - sql

Question

I have 2 columns in a one-to-many relationship. I want to sort on the "many" and return the first occurrence of the "one". I need to page through the data so, for example, I need to be able to get the 3rd group of 10 unique "one" values.
I have a query like this:
SELECT id, name
FROM table1
INNER JOIN table2 ON table2.fkid = table1.id
ORDER BY name, id;
There can be multiple rows in table2 for each row in table1.
The results of my query look like this:
id | name
----------------
2 | apple
23 | banana
77 | cranberry
23 | dark chocolate
8 | egg
2 | yak
19 | zebra
I need to page through the result set with each page containing n unique ids. For example, if start=1 and n=4 I want to get back
2
23
77
8
in the order they were sorted on (i.e., name), where id is returned in the position of its first occurrence. Likewise if start=3 and n=4 and order = desc I want
8
23
77
2
I tried this:
SELECT * FROM (
SELECT id, ROWNUM rnum FROM (
SELECT DISTINCT id FROM (
SELECT id, name
FROM table1
INNER JOIN table2 ON table2.fkid = table1.id
ORDER BY name, id)
WHERE ROWNUM <= 4)
WHERE rnum >=1)
which gave me the ids in numerical order, instead of being ordered as the names would be.
I also tried:
SELECT * FROM (
SELECT DISTINCT id, ROWNUM rnum FROM (
SELECT id FROM (
SELECT id, name
FROM table1
INNER JOIN table2 ON table2.fkid = table1.id
ORDER BY name, id)
WHERE ROWNUM <= 4)
WHERE rnum >=1)
but that gave me duplicate values.
How can I page through the results of this data? I just need the ids, nothing from the "many" table.
update
I suppose I'm getting closer with changing my inner query to
SELECT id, name, rank() over (order by name, id)
FROM table1
INNER JOIN table2 ON table2.fkid = table1.id
...but I'm still getting duplicate ids.

Answer 1

You may need to debug it a little, but but it will be something like this:
SELECT * FROM (
SELECT * FROM (
SELECT id FROM (
SELECT id, name, row_number() over (partition by id order by name) rn
FROM table1
INNER JOIN table2 ON table2.fkid = table1.id
)
) WHERE rn=1 ORDER BY name, id
) WHERE rownum>=1 and rownum<=4;

Answer 2

It's a bit convoluted (and I would tend to suspect that it could be simplified) but it should work. You'd can put whatever start and end position you want in the WHERE clause-- I'm showing here with start=2 and n=4 are pulled from a separate table but you could simplify things by using a couple of parameters instead.
SQL> ed
Wrote file afiedt.buf
1 with t as (
2 select 2 id, 'apple' name from dual union all
3 select 23, 'banana' from dual union all
4 select 77, 'cranberry' from dual union all
5 select 23, 'dark chocolate' from dual union all
6 select 8, 'egg' from dual union all
7 select 2, 'yak' from dual union all
8 select 19, 'zebra' from dual
9 ),
10 x as (
11 select 2 start_pos, 4 n from dual
12 )
13 select *
14 from (
15 select distinct
16 id,
17 dense_rank() over (order by min_id_rnk) outer_rnk
18 from (
19 select id,
20 min(rnk) over (partition by id) min_id_rnk
21 from (
22 select id,
23 name,
24 rank() over (order by name) rnk
25 from t
26 )
27 )
28 )
29 where outer_rnk between (select start_pos from x) and (select start_pos+n-1 from x)
30* order by outer_rnk
SQL> /
ID OUTER_RNK
---------- ----------
23 2
77 3
8 4
19 5