MySQL按时间戳排序预先排序的记录

Question

我遇到了MySQL查询的问题。 我一直在：How to select the most recent set of dated records from a mysql table和我想要做的是选择最recient记录和排序由sales_total字段。所以首先我必须按照max（timestamp）排序，然后再由sales_total进行排序。我已经修改了上面如下给出的解决方案：

$query = "(SELECT * FROM (SELECT * FROM table_name WHERE year =\'2013\' AND category=\'Network\' ORDER BY timestamp DESC) as t1 GROUP BY name) ORDER BY sales_total DESC"; 
$result = mysql_query($query) or die("Query failed"); 

这完全运行在phpMyAdmin，但如果我尝试和PHP脚本，我收到了查询失败的错误执行相同的查询。任何人都知道为什么上述似乎在PHPMyadmin，但不是在PHP中工作？

---

我的PHP查询如下：

$query = "(SELECT * FROM (SELECT * FROM table_name WHERE category=\'Network\' ORDER BY timestamp DESC) as t1 GROUP BY name) ORDER BY sales_total DESC"; 

Answer 1

在你的PHP代码，你有\'尽管使用"引号。此外，你似乎有一些超级（可能是错误）括号。因此，请尝试将您的PHP查询字符串更改为

$query = "SELECT * 
    FROM (
     SELECT * 
     FROM table_name 
     WHERE category='Network' 
     ORDER BY timestamp DESC 
    ) as t1 
    GROUP BY name 
    ORDER BY sales_total DESC"; 

Answer 2

不幸的是，您正在查看的帖子是错误的。 MySQL文档明确指出group by在group by子句中没有指定时为列选择任意值。

以下是来自here的明确引用。

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values within each group the server chooses.

正确的做法（这是标准SQL）是计算适当的最大时间戳记，然后加入结果回：

SELECT t.* 
FROM table_name t join 
     (select name, year, category, max(timestamp) as maxts 
     from table_name 
     WHERE year ='2013' AND category='Network' 
     group by name, year, category 
    ) tn 
     on t.name = tn.name and 
     t.year = tn.year and t.category = tn.category and t.timestamp = tn.maxts 
ORDER BY sales_total DESC; 

编辑：

这是您的查询：

SELECT t.* 
FROM annualtable t join 
    (SELECT year, name, max(countries) 
     FROM annualtable 
     WHERE name NOT LIKE '%No Entries%' and year <> '0' 
     GROUP BY year 
     ORDER BY year 
    ) tn 
    on t.name = tn.name and t.year = tn.year t.countries = tn.countries 
ORDER BY year DESC; 

有两个问题。其中一个是在on的第三个条款之前缺少的and。第二个是你需要在子查询的group by中包含name。如果您希望order by做任何事情，您可能会遇到其他问题。请参阅文档中的上述参考。所以，你可能想要这个：

SELECT t.* 
FROM annualtable t join 
    (SELECT year, name, max(countries) 
     FROM annualtable 
     WHERE name NOT LIKE '%No Entries%' and year <> '0' 
     GROUP BY year, name 
    ) tn 
    on t.name = tn.name and t.year = tn.year and t.countries = tn.countries 
ORDER BY year DESC;