Question

我有一张看起来像这样的桌子。查询需要返回每个位置的最后日期条目。我将如何在SQL和LINQ中编写它？

Id  Location        Salary  Date
1   Los Angeles     $1500   Jan-05-1999 
2   San Diego       $250    Jan-07-1999 
3   Los Angeles     $300    Jan-08-1999
4   Boston          $700    Jan-08-1999
5   Los Angeles     $300    Jan-08-1999

结果将是：

2   San Diego       $250    Jan-07-1999
4   Boston          $700    Jan-08-1999
5   Los Angeles     $300    Jan-08-1999

Answer 1

请记住，我很新的链接，我不认为这是最好的方式：

var collection = from d in <TABLE> select d;
collection = collection.GroupBy(x => x.Date).Select(x => x.Last());

基本上只是在日期组，并采取最后一个。我想不出在LINQ命令中执行此操作的好方法，所以我只选择了所有行 - 任何人都知道将这些行组合起来？

Answer 2

如果您的日期包含时间，那么您可以：

var locations = 
   from l in mycontext.Locations
   where l.Date = mycontext.Locations
         .Where(l2=>l.Location == l2.Location)
         .Max(l2.Date)
   select l;

或者，假设ID是一个标识，并且记录在它们出现时插入到表中（您的示例似乎提示如此），您可以这样做：

var locations = 
   from l in mycontext.Locations
   where l.ID = mycontext.Locations
         .Where(l2=>l.Location == l2.Location)
         .Max(l2.ID)
   select l;

Answer 3

SELECT location, max(date)

FROM yourtable

GROUP BY location

编辑：我没有看到结果被要求或不确定它是否最初存在：

SELECT yourtable.id, yourtable.location, yourtable.salary, yourtable.date
FROM yourtable INNER JOIN
  (SELECT yourtable.location, MAX(yourtable.date) as thedate
   FROM yourtable
   GROUP BY yourtable.location) AS MYRESULT 
   on MYRESULT.thedate = yourtable.date  
      AND MYRESULT.location = yourtable.location

虽然没有测试过。

Answer 4

@Bobby Shaftoe

我相信他正在寻找最新的条目，而不是每个地点的最新日期。如果我的假设是正确的，那就更像是

SELECT max(ID),location,date
FROM tablename
GROUP BY location

Answer 5

要获得每个位置的最大日期的整行，我认为SQL是：

SELECT t1.Id, t1.Location, t1.Salary, t1.Date
FROM YourTable t1, 
    (SELECT Location, MAX(Date) AS MaxDate
    FROM YourTable
    GROUP BY Location) t2
WHERE t1.Location = t2.Location
AND t1.Date = t2.MaxDate

如果我错了，请纠正我！

Answer 6

实际上，如果我正确地阅读这个问题，这不是那么直截了当。由于洛杉矶在最大日期有2个条目，因此似乎还有一个规则，即在这种情况下应选择最大ID。

在这种情况下，SQL变得有点复杂，需要2个分组....

select id, location, salary, date from 
locations where id in
(select  max(id) as MaxID
from locations where date in (
Select Max(Date) as MaxDate from Locations group by Location)
Group by location)

相应的Linq查询，还需要2个分组...

  var qry = from a in locations
            group a by a.Location into grp
            from b in grp
            where b.Date == grp.Max(o => o.Date)
            group b by b.Location into grp2
            from c in grp2
            where c.ID == grp2.Max(obj => obj.ID)
            orderby c.ID
            select c;

如何在LINQ中编写SQL查询？

6 个答案: