Question

How do I convert the following SQL statement into Lambda Expression or Linq Query?

The following query get the single most recent Answer for each Question. Or to phrase it another way, get each Question with the newest Answer.

Also this will be execute by Entity Framework.

SELECT Answers.*
FROM Answers
Where AnswerID IN
(
    SELECT Max(AnswerID) AnswerID
    FROM Answers
    GROUP BY QuestionID
)

Here another way to look at the previous query using an Inner Join

SELECT answers.* 
FROM answers 
INNER JOIN  
(
     SELECT Max(answerID) answerID --,  QuestionSiteID
     FROM answers
     GROUP BY QuestionID 
) t ON
     answers.answerID = t.answerID  

I have read that the LINQ Contains method is sub optimal for queries that access SQL.
LINQ to Sql and .Contains() trap.

Answer 1

I think you could do this using something like:

 var subQuery = from a in answers
                group a by a.QuestionID into grouping
                select new
                {
                    QuestionID = grouping.Key,
                    MaxAnswerID = grouping.Max(x => x.AnswerID)
                };

 var query = from a in answers
             from s in subQuery
             where a.AnswerID == s.MaxAnswerID
             select a;

This results in a CROSS JOIN in the generated SQL

---

Also, you could use join in the second part of the query:

 var query = from a in answers
             join s in subQuery on a.AnswerID equals s.MaxAnswerID
             select a;

This results in a INNER JOIN in the SQL

---

Note for side cases – the above answers make the reasonable assumption that AnswerID is the primary key of Answers – if you happen to have instead a table design which is keyed on (AnswerID, QuestionID) then you will need to join by both AnswerID and QuestionID like:

 var subQuery = from a in answers
                group a by a.QuestionID into grouping
                select new
                {
                    QuestionID = grouping.Key,
                    MaxAnswerID = grouping.Max(x => x.AnswerID)
                };

 var query = from a in answers
             from s in subQuery
             where a.AnswerID == s.MaxAnswerID
             && a.QuestionID == s.QuestionID
             select a;

See the comment trail for more discussion on this alternate table design…