Subqueries oracle - sql

Question

I would like to select house_id, house_name, image_name, and size for the lowest image size for each house.
create table house( id int, name varchar(50));
create table image( id int, name varchar(50), house_id int, siz int);
insert into house (id, name) values (1,'house1');
insert into house (id, name) values (2,'house2');
insert into image values (31,'img1',1, 10);
insert into image values (32,'img2',2, 20);
insert into image values (33,'img3',2, 15);
insert into image values (34,'img4',2, 19);
http://sqlfiddle.com/#!4/f86fe/35
Can anybody help me? I tried to prepare some query, but in the last column I always get the null values.
select h.id, h.name, m.mid, i.name
from house h left join (select min(siz) as mid,house_id from image
group by house_id) m on h.id = m.house_id
left join image i on i.id=m.house_id
Results:
| ID | NAME | MID | NAME |
|----|--------|-----|--------|
| 1 | house1 | 10 | (null) |
| 2 | house2 | 15 | (null) |

Answer 1

Two options - one using ROW_NUMBER() analytic function and the other using aggregation functions:
SQL Fiddle
Oracle 11g R2 Schema Setup:
create table house( id int, name varchar(50));
create table image( id int, name varchar(50), house_id int, siz int);
insert into house (id, name) values (1,'house1');
insert into house (id, name) values (2,'house2');
insert into image values (31,'img1',1, 10);
insert into image values (32,'img2',2, 20);
insert into image values (33,'img3',2, 15);
insert into image values (34,'img4',2, 19);
Query 1:
SELECT h.id, h.name, i.siz, i.name AS image_name
FROM house h
LEFT OUTER JOIN (
SELECT i.*,
ROW_NUMBER() OVER (
PARTITION BY house_id
ORDER BY siz
) AS rn
FROM image i
) i
ON ( h.id = i.house_id AND i.rn = 1 )
Results:
| ID | NAME | SIZ | IMAGE_NAME |
|----|--------|-----|------------|
| 1 | house1 | 10 | img1 |
| 2 | house2 | 15 | img3 |
Query 2:
SELECT h.id,
MIN( h.name ) AS name,
MIN( i.siz ) AS siz,
MIN( i.name ) KEEP ( DENSE_RANK FIRST ORDER BY siz ) AS image_name
FROM house h
LEFT OUTER JOIN image i
ON ( h.id = i.house_id )
GROUP BY h.id
Results:
| ID | NAME | SIZ | IMAGE_NAME |
|----|--------|-----|------------|
| 1 | house1 | 10 | img1 |
| 2 | house2 | 15 | img3 |

Answer 2

In Oracle 12c, you can use OUTER APPLY, which is simpler (in my view) than ROW_NUMBER()
SELECT h.id, h.name, i.siz, i.name as image_name
FROM house h
OUTER APPLY ( SELECT i.*
FROM image i
WHERE i.house_id = h.id
ORDER BY i.siz ASC
FETCH FIRST 1 ROW ONLY ) i;
+----+--------+-----+------------+
| ID | NAME | SIZ | IMAGE_NAME |
+----+--------+-----+------------+
| 1 | house1 | 10 | img1 |
| 2 | house2 | 15 | img3 |
+----+--------+-----+------------+