SQL left join two tables independently - sql

Question

If I have these tables:
Thing
id | name
---+---------
1 | thing 1
2 | thing 2
3 | thing 3
Photos
id | thing_id | src
---+----------+---------
1 | 1 | thing-i1.jpg
2 | 1 | thing-i2.jpg
3 | 2 | thing2.jpg
Ratings
id | thing_id | rating
---+----------+---------
1 | 1 | 6
2 | 2 | 3
3 | 2 | 4
How can I join them to produce
id | name | rating | photo
---+---------+--------+--------
1 | thing 1 | 6 | NULL
1 | thing 1 | NULL | thing-i1.jpg
1 | thing 1 | NULL | thing-i2.jpg
2 | thing 2 | 3 | NULL
2 | thing 2 | 4 | NULL
2 | thing 2 | NULL | thing2.jpg
3 | thing 3 | NULL | NULL
Ie, left join on each table simultaneously, rather than left joining on one than the next?
This is the closest I can get:
SELECT Thing.*, Rating.rating, Photo.src
From Thing
Left Join Photo on Thing.id = Photo.thing_id
Left Join Rating on Thing.id = Rating.thing_id

Answer 1

You can get the results you want with a union, which seems the most obvious, since you return a field from either ranking or photo.
Your additional case (have none of either), is solved by making the joins left join instead of inner joins. You will get a duplicate record with NULL, NULL in ranking, photo. You can filter this out by moving the lot to a subquery and do select distinct on the main query, but the more obvious solution is to replace union all by union, which also filters out duplicates. Easier and more readable.
select
t.id,
t.name,
r.rating,
null as photo
from
Thing t
left join Rating r on r.thing_id = t.id
union
select
t.id,
t.name,
null,
p.src
from
Thing t
left join Photo p on p.thing_id = t.id
order by
id,
photo,
rating

Answer 2

Here's what I came up with:
SELECT
Thing.*,
rp.src,
rp.rating
FROM
Thing
LEFT JOIN (
(
SELECT
Photo.src,
Photo.thing_id AS ptid,
Rating.rating,
Rating.thing_id AS rtid
FROM
Photo
LEFT JOIN Rating
ON 1 = 0
)
UNION
(
SELECT
Photo.src,
Photo.thing_id AS ptid,
Rating.rating,
Rating.thing_id AS rtid
FROM
Rating
LEFT JOIN Photo
ON 1 = 0
)
) AS rp
ON Thing.id IN (rp.rtid, rp.ptid)

Answer 3

MySQL has no support for full outer joins so you have to hack around it using a UNION:
Here's the fiddle: http://sqlfiddle.com/#!2/d3d2f/13
SELECT *
FROM (
SELECT Thing.*,
Rating.rating,
NULL AS photo
FROM Thing
LEFT JOIN Rating ON Thing.id = Rating.thing_id
UNION ALL
SELECT Thing.*,
NULL,
Photo.src
FROM Thing
LEFT JOIN Photo ON Thing.id = Photo.thing_id
) s
ORDER BY id, photo, rating