想要删除 SQL 中的重复项

Question

I am creating an app with rails.
I'm stuck with how to write SQL statements, so please teach me.

want to

From this, narrow down by "competition_id" and extract only one item with the smallest "id".

For example,

+----------------+----+---------------------------+----------------+-------+-----+
| competition_id | id | image                     | name           | count | rnk |
+----------------+----+---------------------------+----------------+-------+-----+
|              1 |  4 | monster4.jpeg             | monster4       |     7 |   1 |
|              2 |  6 | monster2.jpeg             | monster2       |     1 |   1 |
|              3 |  9 | monster1.jpeg             | monster1       |     1 |   1 |
|              5 | 22 | drink_sample.jpeg         | drink6         |     2 |   1 |
|              6 | 33 | sumo_wrestler_sample.jpeg | sumo_wrestler7 |     2 |   1 |
|              7 | 40 | movie_sample.jpeg         | movie4         |     2 |   1 |
|              8 | 50 | food_sample.jpeg          | food4          |     2 |   1 |
|              9 | 61 | color_sample.jpeg         | color5         |     3 |   1 |
|             10 | 72 | book_sample.jpeg          | book6          |     2 |   1 |
|             11 | 82 | book_sample.jpeg          | book6          |     3 |   1 |
+----------------+----+---------------------------+----------------+-------+-----+

Current status

+----------------+----+---------------------------+----------------+-------+-----+
| competition_id | id | image                     | name           | count | rnk |
+----------------+----+---------------------------+----------------+-------+-----+
|              1 |  4 | monster4.jpeg             | monster4       |     7 |   1 |
|              2 |  6 | monster2.jpeg             | monster2       |     1 |   1 |
|              3 |  9 | monster1.jpeg             | monster1       |     1 |   1 |
|              5 | 22 | drink_sample.jpeg         | drink6         |     2 |   1 |
|              6 | 33 | sumo_wrestler_sample.jpeg | sumo_wrestler7 |     2 |   1 |
|              6 | 34 | sumo_wrestler_sample.jpeg | sumo_wrestler8 |     2 |   1 |
|              6 | 35 | sumo_wrestler_sample.jpeg | sumo_wrestler9 |     2 |   1 |
|              7 | 40 | movie_sample.jpeg         | movie4         |     2 |   1 |
|              7 | 43 | movie_sample.jpeg         | movie7         |     2 |   1 |
|              7 | 45 | movie_sample.jpeg         | movie9         |     2 |   1 |
|              8 | 50 | food_sample.jpeg          | food4          |     2 |   1 |
|              8 | 56 | food_sample.jpeg          | food10         |     2 |   1 |
|              9 | 61 | color_sample.jpeg         | color5         |     3 |   1 |
|             10 | 72 | book_sample.jpeg          | book6          |     2 |   1 |
|             11 | 82 | book_sample.jpeg          | book6          |     3 |   1 |
+----------------+----+---------------------------+----------------+-------+-----+

The code that extracted this

SELECT * FROM (SELECT *,RANK() OVER (PARTITION BY competition_id ORDER BY COUNT DESC) rnk FROM (SELECT items.competition_id,items.id,items.image,items.name,count(*) AS count FROM chosenitems INNER JOIN items ON chosenitems.item_id = items.id GROUP BY items.competition_id,items.id) AS t) AS tt WHERE rnk = 1;

Table structure

• chosenitems table

id	session_id	item_id
1	1	2
2	1	3
2	1	2
2	1	2
2	1	5
3	1	7
4	1	4
5	1	5

• items table

id	name	image	competition_id
1	a	image1	1
2	b	image2	1
2	c	image2	1
2	d	image2	1
2	e	image5	2
3	f	image9	2
4	g	image4	2
5	h	image5	2

I tried

SELECT * FROM (SELECT *,RANK() OVER (PARTITION BY competition_id ORDER BY COUNT DESC) rnk FROM (SELECT items.competition_id,items.id,items.image,items.name,count(*) AS count FROM chosenitems INNER JOIN items ON chosenitems.item_id = items.id GROUP BY items.competition_id,items.id) AS t) AS tt WHERE id in (SELECT MIN(id) FROM chosenitems GROUP BY competition_id);

↓

Failure

+----------------+----+---------------+----------+-------+-----+
| competition_id | id | image         | name     | count | rnk |
+----------------+----+---------------+----------+-------+-----+
|              1 |  1 | monster1.jpeg | monster1 |     4 |   2 |
+----------------+----+---------------+----------+-------+-----+

I want to extract the one with the smallest id from each "competition_id" where "rnk" is 1.

environment

macOS BigSur
ruby 2.7.0
Rails 6.1.1
mysql Ver 8.0.23

Answer 1

This should give you the wanted result, you must check if MIN is the right choice for every value

SELECT 
    competition_id
    ,MIN(id) as id
    ,MIN(mage) as image
    ,MIN(ame) as name
    ,MIN(count) as count
   ,MIN(rnk) as rnk
FROM 
    (SELECT 
        *
        ,RANK() OVER (PARTITION BY competition_id ORDER BY COUNT DESC) rnk 
    FROM (SELECT 
            items.competition_id
            ,items.id
            ,items.image
            ,items.name
            ,count(*) AS count 
        FROM 
            chosenitems INNER JOIN items ON chosenitems.item_id = items.id 
            GROUP BY items.competition_id,items.id) AS t) AS tt 
WHERE rnk = 1
GROUP BY competition_id;