Linq to sql string.Join（）中的不同值

Question

I have a table which contains the schedule for a class. it contains the teacherid and name, datetime of the class and the course for that specific time.

teacherid    teachername    startdate         coursename
1            john           9/1/2014 10:00    math
2            john           9/2/2014 10:00    math
3            jane           9/3/2014 10:00    english
4            john           9/4/2014 10:00    french
5            jack           9/5/2014 10:00    history
6            jane           9/6/2014 10:00    math

I want to write a linq to sql query which returns which teacher gives which courses, as such:

Teachername    courses
john           math, french
jane           english, math
jack           history

I've gotten as far as the following

var _classes = from _c in dbContext.classes                           
                       where _c.StartDate < System.DateTime.Now
                       group _c by new
                       {
                           _c.TeacherId,
                           _c.TeacherName,
                           _c.CourseName
                       } into g
                       select new
                       {
                           g.Key.TeacherName,
                           courses = string.Join(",", from i in g select i.CourseName)
                       };

but i'm getting the following output

Teachername    courses
john           math, math, french
jane           english, math
jack           history

so john is getting the math value twice. how do I make the string.Join function use distinct values?

thanks

Answer 1

没有用于distinct查询语法，因此您应该更改为方法语法：

courses = string.Join(",", g.Select(i => i.CourseName).Distinct())

Answer 2

Add Distinct to your select clause in String.Join like:

courses = string.Join(",", (from i in g select i.CourseName).Distinct())

Or use method Syntax for select as well like:

courses = string.Join(",", (g.Select(i=> i.CourseName).Distinct())