在MySQL中返回每个'group by'的'last'行

Question

有更有效的方法来做以下事情吗？

select * 
    from foo as a
    where a.id = (select max(id) from foo where uid = a.uid group by uid)
    group by uid;
)

这个答案看起来很相似,但这个答案是最好的方法 - 如何为MySQL中的每个组选择第一行？

谢谢,

克里斯.

PS表格如下:

CREATE TABLE foo (
    id INT(10) NOT NULL AUTO_INCREMENT,
    uid INT(10) NOT NULL,
    value VARCHAR(50) NOT NULL,
    PRIMARY KEY (`id`),
    INDEX `uid` (`uid`)
)

数据:

id, uid, value
 1,   1, hello
 2,   2, cheese
 3,   2, pickle
 4,   1, world

结果:

id, uid, value
 3,   2, pickle
 4,   1, world

有关详细信息,请参阅http://www.barricane.com/2012/02/08/mysql-select-last-matching-row.html.

Answer 1

试试这个查询 -

SELECT t1.* FROM foo t1
  JOIN (SELECT uid, MAX(id) id FROM foo GROUP BY uid) t2
    ON t1.id = t2.id AND t1.uid = t2.uid;

然后使用EXPLAIN来分析查询.

SELECT t1.* FROM foo t1
  LEFT JOIN foo t2
    ON t1.id < t2.id AND t1.uid = t2.uid
WHERE t2.id is NULL;

Answer 2

GROUP BY使用WHERE子句返回MySQL中每个的最后一行：

SELECT *
FROM foo
WHERE id IN (
  SELECT Max(id)
  FROM foo
  WHERE value='XYZ'
  GROUP BY u_id
)
LIMIT 0,30