How to reduce join operation to a single row in Oracle? - oracle

Question

This example is invented for the purpose of the question.
SELECT
PR.PROVINCE_NAME
,CO.COUNTRY_NAME
FROM
PROVINCE PR
JOIN COUNTRY CO ON CO.COUNTRY_ID=PR.COUNTRY_ID
WHERE
PR.PROVINCE_ID IN (1,2)
Let's assume that COUNTRY_ID is not the Primary Key in the Country table and the above join on Country table returns potentially multiple rows. We don't know how many rows and we don't care why there are multiple ones. We only want to join on one of them, so we get one row per Province.
I tried subquery for the join but can't pass in PR.COUNTRY_ID for Oracle 11.2. Are there any other ways that this can be achieved?

Answer 1

A typical safe approach of handling tables without PK is to extend the duplicated column with a unique index (row_numer of the duplicated row)
In your case this would be:
with COUNTRY_UNIQUE as (
select COUNTRY_ID,
row_number() over (partition by COUNTRY_ID order by COUNTRY_NAME) rn,
COUNTRY_NAME
from country)
select * from COUNTRY_UNIQUE
order by COUNTRY_ID, rn;
leading to
COUNTRY_ID RN COUNTRY_NAME
---------- ---------- ------------
1 1 C1
2 1 C2
2 2 C3
The combination of COUNTRY_IDand RN is unique, so if you constraint only RN = 1 the COUNTRY_ID is unique.
You may define the order of the duplicated records and control with it the selection - in our case we choose the smalest COUNTRY_NAME.
The whole join used this subquery and constraints the countries on RN = 1
with COUNTRY_UNIQUE as (
select COUNTRY_ID,
row_number() over (partition by COUNTRY_ID order by COUNTRY_NAME) rn,
COUNTRY_NAME
from country)
SELECT
PR.PROVINCE_NAME
,CO.COUNTRY_NAME
FROM
PROVINCE PR
JOIN COUNTRY_UNIQUE CO ON CO.COUNTRY_ID=PR.COUNTRY_ID
WHERE
PR.PROVINCE_ID IN (1,2)
AND CO.RN = 1; /* consider only unique countries */

Answer 2

If you have Oracle 12c, you can use a LATERAL view in the join. Like this:
SELECT
PR.PROVINCE_NAME
,CO.COUNTRY_NAME
FROM
PROVINCE PR
CROSS JOIN LATERAL (
SELECT * FROM COUNTRY CO
WHERE CO.COUNTRY_ID=PR.COUNTRY_ID
FETCH FIRST 1 ROWS ONLY) CO
WHERE
PR.PROVINCE_ID IN (1,2)
Update for Oracle 11.2
In Oracle 11.2, you can use something along these lines. Depending on the size of COUNTRY and how many duplicates there are per COUNTRY_ID, it could perform as well or better than the 12c approach. (Fewer buffer gets but more memory required).
SELECT pr.province_name,
co.country_name
FROM province pr
INNER JOIN (SELECT *
FROM (SELECT co.*,
ROW_NUMBER () OVER (PARTITION BY co.country_id ORDER BY co.country_name) rn
FROM country co)
WHERE rn = 1) co
ON co.country_id = pr.country_id
WHERE pr.province_id IN (1, 2)