LINQ Join with Multiple From Clauses

Question

When writing LINQ queries in C#, I know I can perform a join using the join keyword. But what does the following do?

from c in Companies from e in c.Employees select e; 

A LINQ book I have say it's a type of join, but not a proper join (which uses the join keyword). So exactly what type of join is it then?

Answer 1

Multiple "from" statements are considered compound linq statments. They are like nested foreach statements. The msdn page does list a great example here

var scoreQuery = from student in students                  from score in student.Scores                  where score > 90                  select new { Last = student.LastName, score }; 

this statement could be rewritten as:

SomeDupCollection<string, decimal> nameScore = new SomeDupCollection<string, float>(); foreach(Student curStudent in students) {    foreach(Score curScore in curStudent.scores)    {       if (curScore > 90)       {          nameScore.Add(curStudent.LastName, curScore);       }    } } 

Answer 2

This will get translated into a SelectMany() call. It is essentially a cross-join.

Jon Skeet talks about it on his blog, as part of the Edulinq series. (Scroll down to Secondary "from" clauses.)