How to select only the records with the highest date in LINQ

I have a table, 'lasttraces', with the following fields.

Id, AccountId, Version, DownloadNo, Date

The data looks like this:

28092|15240000|1.0.7.1782|2009040004731|2009-01-20 13:10:22.000
28094|61615000|1.0.7.1782|2009040007696|2009-01-20 13:11:38.000
28095|95317000|1.0.7.1782|2009040007695|2009-01-20 13:10:18.000
28101|15240000|1.0.7.1782|2009040004740|2009-01-20 14:10:22.000
28103|61615000|1.0.7.1782|2009040007690|2009-01-20 14:11:38.000
28104|95317000|1.0.7.1782|2009040007710|2009-01-20 14:10:18.000

How can I, in LINQ to SQL, only get the last lasttrace of every AccountId (the one with the highest date)?

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最佳答案

If you just want the last date for each account, you'd use this:

var q = from n in table
group n by n.AccountId into g
select new {AccountId = g.Key, Date = g.Max(t=>t.Date)};

If you want the whole record:

var q = from n in table
group n by n.AccountId into g
select g.OrderByDescending(t=>t.Date).FirstOrDefault();
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It could be something like:

var qry = from t in db.Lasttraces
group t by t.AccountId into g
orderby t.Date
select new { g.AccountId, Date = g.Max(e => e.Date) };
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Go a simple way to do this :-

Created one class to hold following information

  • Level (number)
  • Url (Url of the site)

Go the list of sites stored on a ArrayList object. And executed following query to sort it in descending order by Level. 

var query = from MyClass object in objCollection
orderby object.Level descending
select object

Once I got the collection sorted in descending order, I wrote following code to get the Object that comes as top row

MyClass topObject = query.FirstRow<MyClass>()

This worked like charm.

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Here is a simple way to do it

var lastPlayerControlCommand = this.ObjectContext.PlayerControlCommands
.Where(c => c.PlayerID == player.ID)
.OrderByDescending(t=>t.CreationTime)
.FirstOrDefault();

Also have a look this great LINQ place - LINQ to SQL Samples

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If you want the whole record,here is a lambda way:

var q = _context
.lasttraces
.GroupBy(s => s.AccountId)
.Select(s => s.OrderByDescending(x => x.Date).FirstOrDefault());
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LINQ's .OrderByDescending().FirstOrDefault() approach is good. But the https://github.com/morelinq/MoreLINQ people do it better: .MaxBy()

Example:

IEnumerable<Person> data = Data.GetPersons();
Person oldestPerson = data.MaxBy(p => p.Age);

Or in the latest version of MoreLINQ, they have changed the return type:

IEnumerable<Person> data = Data.GetPersons();
IEnumerable<Person> oldestPersons = data.MaxBy(p => p.Age);
Person oldestPerson = oldestPersons.First();

So what used to be .MaxBy() is now .MaxBy().First(). This has the advantage that you can handle more than 1 result (used to be only the first person with the max age, but there may be more persons with the same age). Another advantage is that .MaxBy() can now handle an empty IEnumerable<T> without throwing an exception. Of course, the exception occurs in .First() then, but you can decide to use .FirstOrDefault() instead to avoid this.


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