Remove duplicates based on rank after join in SQL request - mysql

Question

I am using MySQL 5.6.
I have a SQL table with a list of users:
id name
1 Alice
2 Bob
3 John
and a SQL table with the list of gifts for each user (numbered in order of preference):
id gift rank
1 balloon 2
1 shoes 1
1 seeds 3
1  video-game 1
2 computer 2
3 shoes 2
3 hat 1
And I would like a list of the preferred gift for each user (the highest rank - if two gifts have the same rank, pick only one randomly) (bonus: if the list could be randomized, that would be perfect!):
id name gift rank
2 Bob computer 2
1 Alice shoes 1
3 John hat 1
I tried to use the clause GROUP BY but without any success.

Answer 1

Considering rank as a part of your data; Without using window functions or complex sub queries
SELECT u.id, u.name, g.gift
FROM users u
JOIN gifts g ON g.id = u.id
LEFT JOIN gifts g2 ON g2.id = g.id AND g2.rank > g.rank
WHERE g2.id IS NULL;
Added link http://sqlfiddle.com/#!9/62f59e/15/0

Answer 2

You can use row_number to get one row for each User.(Mysql 8.0+)
SELECT A.ID,NAME,GIFT,`RANK` FROM USERS A
LEFT JOIN (
SELECT ID,GIFT,`RANK` FROM
(SELECT *,ROW_NUMBER() OVER(PARTITION BY ID ORDER BY `RANK` ASC) AS RN FROM X) X
WHERE RN =1
) B
ON A.ID= B.ID

Answer 3

I do not know DB what you use. And I'm not an expert in SQL(I can have some mistake in next). But I think it is not difficult.
So I can give you just advice that you have to think gradually. Let me write.
First All I need is the highest rank. So I have to get this.
SELECT MAX(RANK)
FROM GIFT
GROUP BY ID
And then I think that I need get gifts from this rank.
SELECT GIFT.*
FROM GIFT
INNER JOIN(
SELECT ID, MAX(RANK)
FROM GIFT
GROUP BY ID
) filter ON GIFT.ID = filter.ID AND GIFT.RANK = filter.RANK
I think this is the table what you want!
So If below code works, That's what you really want.
SELECT *
FROM USER
LEFT OUTER JOIN(
above table
) GIFT ON USER.ID = GIFT.ID
But Remember this, I said I'm not an expert in SQL. There can be better way.

Answer 4

Checkout the query
SELECT tbluser.id,name,gift,rank into tblrslt
FROM tbluser
LEFT JOIN tblgifts
ON tbluser.id = tblgifts.id order by id,rank;
SELECT tt.*
FROM tblrslt tt
INNER JOIN
(SELECT id, min(rank) AS rank
FROM tblrslt
GROUP BY id) groupedtt
ON tt.id = groupedtt.id
AND tt.rank = groupedtt.rank order by id

Answer 5

In MySQL versions older than 8 you have no ranking functions available. You'll select the minimum rank per user instead and use these ranks to select the gift rows. This means you access the gifts table twice.
I suggest this:
select *
fron users u
join gifts g
on g.id = u.id
and (g.id, g.rank) in (select id, min(rank) from gifts group by id)
order by u.id;
If you also want to show users without gifts, simply change the inner join to a left outer join.