MySQL排序DESC和ASC

Question

I have a table with the fields:

id | group_id
1  | null
2  | null
3  | null
4  | 4
5  | 4
6  | 4
7  | 7
8  | 7
9  | null

and I'd like to sort it to be:

id | group_id
9  | null
7  | 7
8  | 7
4  | 4
5  | 4
6  | 4
3  | null
2  | null
1  | null

So for it to be DESC by id overall, but ASC by id when they share a group_id

Thank you

Answer 1

试试吧

Select id, group_id, coalesce(group_id, id) from table order by 3 desc, 1

Answer 2

I got this to work:

SELECT id, group_id, rank
FROM (
  SELECT id, group_id, 
    @rank:=IF(group_id=@group_id,@rank,@rank+1) AS rank,
    @group_id:=group_id
  FROM MyTable CROSS JOIN (SELECT @rank:=0) AS _init
  ORDER BY id DESC
) AS t
ORDER BY rank ASC, id ASC;

Output:

+----+----------+------+
| id | group_id | rank |
+----+----------+------+
|  9 |     NULL | 1    |
|  7 |        7 | 2    |
|  8 |        7 | 2    |
|  4 |        4 | 3    |
|  5 |        4 | 3    |
|  6 |        4 | 3    |
|  3 |     NULL | 4    |
|  2 |     NULL | 5    |
|  1 |     NULL | 6    |
+----+----------+------+

You can omit the rank column from the select-list of the outer query, I left it in there just to illustrate how I ranked the groups.

Answer 3

I'm not sure you can do the out-of-sequence ordering of the id column, but if what you need is the lowest id where the group id is shared, then updating @Lamar's answer like this would do it

SELECT MIN(id) AS id, group_id FROM table
GROUP BY group_id
ORDER BY IFNULL(group_id, 0) DESC, id DESC