问题:

I spend a few hours trying to translate simple SQL to lambda LINQ

SELECT ID, AVG(Score) FROM myTable
GROUP BY ID

Any idea?

回答1:

from t in myTable
group t by new {
  t.ID
} into g
select new {
  Average = g.Average(p => p.Score),
  g.Key.ID
}

or Lambda

myTable.GroupBy(t => new  {ID = t.ID})
   .Select (g => new {
            Average = g.Average (p => p.Score), 
            ID = g.Key.ID 
         })

回答2:

The equivalent in Linq-to-Objects would be something like the below.

var results = from row in myTable
              group row by row.Id into rows 
              select new 
              {
                  Id = rows.Key,
                  AverageScore = rows.Average(row => row.Score)
              };

It's only slightly different for an ORM like entity framework. Namely, you would need to go through the data context or an appropriate DbSet/ObjectSet.

回答3:

var _result =   from a in myTable
                group a by a.ID into g
                select new
                {
                    ID = g.Key.ID,
                    AverageResult = g.Average(x => x.Score)
                }