Question

考虑以下问题：

SELECT domain, done FROM tasks WHERE 1 GROUP BY domain

显然我可以用MAX（完成）返回“完成”的最大值，但是如何让它返回整行呢？

在我所处的情况下，我有一个包含以下行的表：

domain = somedomain.com, done = 12:27:39, somecolumn = pink
domain = somedomain.com, done = 13:27:39, somecolumn = blue
domain = somedomain.com, done = 14:27:39, somecolumn = orange
domain = anotherdomain.com, done = 07:27:39, somecolumn = orange
domain = anotherdomain.com, done = 09:27:39, somecolumn = pie

使用上面的查询，它返回以下2行：

domain = somedomain.com, done = 13:27:39, somecolumn = blue
domain = anotherdomain.com, done = 09:27:39, somecolumn = pie

我需要返回这两行：

domain = somedomain.com, done = 14:27:39, somecolumn = blue
domain = anotherdomain.com, done = 09:27:39, somecolumn = pie

感谢帮助：）。

Answer 1

最简单的一个：

SELECT done,somecolumn FROM tasks ORDER BY done DESC LIMIT 1;

对于更复杂的查询，可能需要左连接或子查询，但这个很简单。

对于它，如果你确实需要其他数据：

（1）NOT EXISTS：

 SELECT a.*
 FROM tasks a
 WHERE NOT EXISTS(
    SELECT * FROM tasks b
    WHERE b.domain = a.domain AND b.done > a.done);

（2）LEFT JOIN

 SELECT a.*
 FROM tasks a
 LEFT JOIN tasks b
 ON b.domain = a.domain AND b.done > a.done
 WHERE b.id IS NULL;

（3）ROWNUMBER（）喜欢：

SELECT numbered.*
FROM (
    SELECT 
        @row := IF(@old_domain = domain,@row + 1,1) as rownumber,
        @old_domain := domain as domain,
        done,
        somecolumn
    FROM tasks
    JOIN (SELECT @old_domain:=0,@row:=0) void
    ORDER BY domain ASC, done DESC
) numbered
WHERE numbered.rownumber = 1;

哪一个会给你带来更好的性能取决于数据集，所以我只是测试它们。

Answer 2

SELECT * FROM tasks WHERE done = (SELECT Max(done) FROM tasks)

Answer 3

select
    *
from
    tasks
where
    `done`=(select max(`done`) from tasks)

选择在GROUP BY行返回哪个结果

3 个答案: