如何编写此linq查询？

Question

I have this code that totally works fine. But I would like to understand how to write it as a single linq query rather than a ling query, and a for loop containing another repeating query.

The idea is that I am looking at database of classes for which people have not completed their work. In the first query, I am finding all the people in the database uniquely. Then in the second query in the for loop, I am iterating though all the names from the first query and counting how many classes they still have left to complete.

Again, the code works, I just wondering the best way to consolidate it into one single query.

        var ListOfNames = (from record in records select record.LastName).Distinct();
        foreach (var NameOfPerson in ListOfNames)
        {
            Console.WriteLine(NameOfPerson);
            var NotCompletedCount = (from rec in records
                                     where rec.LastName == NameOfPerson 
                                     && rec.AssignmentType == "MANDATORY" 
                                     && string.IsNullOrEmpty(rec.CompleteDate) == true
                                     select rec).Count();
            Console.WriteLine(NotCompletedCount);
            rankList.Add(new Rank{ LastName = NameOfPerson, RankCount = NotCompletedCount});
        }

Answer 1

I'm not sure that there are differences between previous answer ant this. In this case we firstly filter data and then Group by name and Select Rank instances. I think in the result .Net will optimize and make the same query to the DB.

var result = records.Where(x => x.AssigmentType == "MANDATORY" && 
string.IsNullOrEmpty(x.CompleteDate))
    .GroupBy(x => x.LastName)
    .Select(group => new Rank {LastName = group.Key, RankCount = group.Count()});

More details how GroupBy works is here: LINQ with groupby and count

Answer 2

As I understand what you are doing is basically for each last name, get the count of records that meets some condition. GroupBy can be used here:

var rankList = records.GroupBy(r => r.LastName)
                      .Select(g => new Rank { 
                                        LastName = g.Key, 
                                        RankCount = g.Count(x => x.AssignmentType == "MANDATORY" && string.IsNullOrEmpty(x.CompleteDate))).ToList()

Sorry the formatting sucks because the Count call is bit long.

This combines your foreach loop and the first query together. It groups records by LastName , creates a Rank instance for each name calculating the count of records that meets your criteria for that name.