按LINQ实体框架分组

Question

I am having trouble grouping a data retrieved from database which has duplication due to a column with data i don't need. i was trying to use distinct or group by to get rid of the redundancies.

var students= db.student.Distinct().Where(c => c.classdate== tomorrow || c.classdate== todayd).ToList();

it will return student's name but also duplicates student record according to course available on that day. I just wanted to get list of students who has class today and tomorrow.

What I have tried:

students.GroupBy(x => new { x.Fn  ame, x.classdate});

The above seems to return result set with only the group by columns and

@model IEnumerable

does not seem to be compatible with the return type.

Answer 1

If you want the first student of your group, you can use this LINQ query:

students.GroupBy(x => new { x.Fname, x.classdate}).Select(grp => grp.First);

This will yield a generic IEnumerable<T> where T is your stident type.

Answer 2

From what I understand, you want to get all the student names who have classes either today or tomorrow. Your LINQ expression is not accurate if that is the case. Your expression should do filter, select and distinct operations in that order. Something like this:

db.student.Where(<condition here>).Select(<columns to select here>).Distinct(<comparer here; optional>)

Answer 3

for "distinct by" you can grouping, and pick the fist row from each group.

so in model:

return students.GroupBy(x => new { x.Fname, x.classdate}).Select(x => x.First()).ToList();

in view:

@model IEnumerable<student>

Answer 4

Just select the column from student table as you want and use Distinct(), like wise your query should be

var students = (from objSql in db.student.Where(t => t.classdate == tomorrow || t.classdate == todayd) select new CustomObject() { StudentName = objSql.StudentName }).Distinct().ToList();

Answer 5

I think you should like this.

context.Student.Select(i => new { i.Fn, i.classdate }).Where(c => c.classdate == tomorrow || c.classdate == todayd).Distinct();

If you run this code, it generate following query in Database.

exec sp_executesql N'SELECT 
    [Distinct1].[C1] AS [C1], 
    [Distinct1].[Fn] AS [Fn], 
    [Distinct1].[classdate] AS [classdate]
    FROM ( SELECT DISTINCT 
        [Extent1].[Fn] AS [Fn], 
        [Extent1].[classdate] AS [classdate], 
        1 AS [C1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE [Extent1].[classdate] IN (@p__linq__0,@p__linq__1)
    )  AS [Distinct1]',N'@p__linq__0 datetime2(7),@p__linq__1 datetime2(7)',@p__linq__0='2018-04-17 00:00:00',@p__linq__1='2018-04-16 00:00:00'