Order result form Mysql group by

问题:

Ok i got this table

+-----+-----------------------------+-------------+-------------+----------+---------+
| id  | title                       | created     | updated     | category | content |
+-----+-----------------------------+---------------------+---------------------+----+
| 423 | What If I Get Sick and Die? | 2008-12-30  | 2009-03-11  | angst    | NULL    |
| 524 | Uncle Karl and the Gasoline | 2009-02-28  | NULL        | humor    | NULL    |
| 537 | Be Nice to Everybody        | 2009-03-02  | NULL        | advice   | NULL    |
| 573 | Hello Statue                | 2009-03-17  | NULL        | humor    | NULL    |
| 598 | The Size of Our Galaxy      | 2009-04-03  | NULL        | science  | NULL    |
+-----+-----------------------------+---------------------+---------------------+----+

I try to get unique row for each category, so i query fo something like:

SELECT id,title,category FROM `entries` group by category

And the result is :

+-----+-----------------------------+----------+
| id  | title                       | category |
+-----+-----------------------------+----------+
| 537 | Be Nice to Everybody        | advice   |
| 423 | What If I Get Sick and Die? | angst    |
| 524 | Uncle Karl and the Gasoline | humor    |
| 598 | The Size of Our Galaxy      | science  |
+-----+-----------------------------+----------+

The result looks fine, but how can i get row with category = humor and id = 573 instead?

If i query something like:

SELECT id,title,category, max(id) FROM `entries` group by category

I got result like this:

+-----+-----------------------------+----------+---------+
| id  | title                       | category | max(id) |
+-----+-----------------------------+----------+---------+
| 537 | Be Nice to Everybody        | advice   |     537 |
| 423 | What If I Get Sick and Die? | angst    |     423 |
| 524 | Uncle Karl and the Gasoline | humor    |     573 |
| 598 | The Size of Our Galaxy      | science  |     598 |
+-----+-----------------------------+----------+---------+

Obviously thats not what i want, any help would be appreciated

And i want to know how exactly mysql collapse row for each group? Did mysql simply take first row for each group?

回答1:

• Using JOIN

and a query with GROUP BY to get maximum id for every category (I guess you do want the row with maximum id for every category, don't you?)

SELECT e.id
     , e.title
     , e.created
     , e.updated
     , e.category
     , e.content
FROM entries e
JOIN
    ( SELECT max(id) AS maxid
      FROM entries
      GROUP BY category
    ) AS cat
ON e.id = cat.maxid

• Using IN

and a query to get maximum id for every category

SELECT id
     , title
     , created
     , updated
     , category
     , content
FROM entries
WHERE id IN
    ( SELECT max(id)
      FROM entries
      GROUP BY category
    )

• Using ANY

and a correlated subquery

SELECT e.id
     , e.title
     , e.created
     , e.updated
     , e.category
     , e.content
FROM entries e
WHERE e.id >= ANY
    ( SELECT cat.id
      FROM entries cat
      WHERE e.category = cat.category
    )

• Using NOT EXISTS

and a correlated subquery

SELECT e.id
     , e.title
     , e.created
     , e.updated
     , e.category
     , e.content
FROM entries e
WHERE NOT EXISTS
    ( SELECT 1
      FROM entries cat
      WHERE cat.id > e.id
        AND e.category = cat.category
    )

回答2:

I think this is what you want. I haven't tested it though.

select * from entries e
inner join (select max(id), category from entries group by category) ids
on e.id=ids.id