equivalent LINQ query - sql-server-2008

Question

I have a history table for Students in SQL Server 2008.
StudentHistoryId, StudentId, Grade, CreatedAt, ModifiedAt, ModifiedBy, Active
I am new to LINQ.
How do I write a LINQ query to get the latest modified row for all the active students and also the equivalent sql query for the same ?

Answer 1

Something like (Assuming LINQ-SQL):
using (YourDataContext db = new YourDataContext())
{
var data = from s in db.Students
select new
{
StudentId = s.StudentId,
LastHistory = s.Histories
.OrderByDescending(s => s.ModifiedAt)
.Where(s => s.Active)
.FirstOrDefault()
};
}
This is assuming that you want all students, regardless of whether they actually have any history. If don't want this, you can start with the History table and group by Student ID.
To view the SQL, you can hover the variable in debugging to see the SQL produced. I'm too lazy to convert the LINQ ;-)

Answer 2

var q =
from h in history
where h.Active
group h by new { h.StudentId, h.Grade } into g
select new
{
StudentId = g.Key.StudentId,
Grade = g.Key.Grade,
LatestModified = g.Max (x => x.ModifiedAt)
}

Answer 3

LINQ
var tempresult = (from student in Students
where Active == true).OrderByDesc(ModifiedAt)
List<Student> results = new List<Student>();
foreach(var result in tempResult)
{
if((results.Where(r => r.StudentId == result.StudentId).FirstOrDefault()) == null)
{
results.Add(result);
}
}
SQL
Select [Rows]
From Students S
Where S.Active = 1
And S.ModifiedAt = (Select Max(ModifiedAt)
From Student S1
Where S1.StudentId = S.StudentId)
The Linq is hacky (and I'm sure there's a better way, but I can't remember it) and I'm only sort-of confident about the SQL syntax (though that should point you in the right direction even if it's not exactly right), but either of those should get: The maximum ModifiedAt for every student that is currently active.
.FirstOrDefault() [LINQ] or Top 1 would only select the single row (only one student) with the most recent ModifiedAt.