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Can someone please explain what a GroupJoin() is?

How is it different from a regular Join() ?

Is it commonly used?

Is it only for method syntax? What about query syntax? (A c# code example would be nice)

When you GroupJoin the two lists on the Id field the result will be:

Value  ChildValues
A      [a1, a2, a3]
B      [b1, b2]
C      []
So Join produces a flat (tabular) result of parent and child values.

GroupJoin produces a list of entries in the first list, each with a group of joined entries in the second list.
That's why Join is the equivalent of INNER JOIN in SQL: there are no entries for C. While GroupJoin is the equivalent of OUTER JOIN: C is in the result set, but with an empty list of related entries (in an SQL result set there would be a row C - null).
Syntax
So let the two lists be IEnumerable<Parent> and IEnumerable<Child> respectively. (In case of Linq to Entities: IQueryable<T>).
Join syntax would be
from p in Parent
join c in Child on p.Id equals c.Id
select new { p.Value, c.ChildValue }
returning an IEnumerable<X> where X is an anonymous type with two properties, Value and ChildValue. This query syntax uses the Join method under the hood.
GroupJoin syntax would be
from p in Parent
join c in Child on p.Id equals c.Id into g
select new { Parent = p, Children = g }
returning an IEnumerable<Y> where Y is an anonymous type consisting of one property of type Parent and a property of type IEnumerable<Child>. This query syntax uses the GroupJoin method under the hood.
We could just do select g in the latter query, which would select an IEnumerable<IEnumerable<Child>>, say a list of lists. In many cases the select with the parent included is more useful.
Some use cases
1. Producing a flat outer join.
As said, the statement ...
from p in Parent
join c in Child on p.Id equals c.Id into g
select new { Parent = p, Children = g }
... produces a list of parents with child groups. This can be turned into a flat list of parent-child pairs by two small additions:
from p in parents
join c in children on p.Id equals c.Id into g // <= into
from c in g.DefaultIfEmpty()               // <= flattens the groups
select new { Parent = p.Value, Child = c?.ChildValue }
The result is similar to
Value Child
A     a1
A     a2
A     a3
B     b1
B     b2
C     (null)
Note that the range variable c is reused in the above statement. Doing this, any join statement can simply be converted to an outer join by adding the equivalent of into g from c in g.DefaultIfEmpty() to an existing join statement.
This is where query (or comprehensive) syntax shines. Method (or fluent) syntax shows what really happens, but it's hard to write:
parents.GroupJoin(children, p => p.Id, c => c.Id, (p, c) => new { p, c })
       .SelectMany(x => x.c.DefaultIfEmpty(), (x,c) => new { x.p.Value, c?.ChildValue } )
So a flat outer join in LINQ is a GroupJoin, flattened by SelectMany.
2. Preserving order
Suppose the list of parents is a bit longer. Some UI produces a list of selected parents as Id values in a fixed order. Let's use:
var ids = new[] { 3,7,2,4 };
Now the selected parents must be filtered from the parents list in this exact order.
If we do ...
var result = parents.Where(p => ids.Contains(p.Id));
... the order of parents will determine the result. If the parents are ordered by Id, the result will be parents 2, 3, 4, 7. Not good. However, we can also use join to filter the list. And by using ids as first list, the order will be preserved:
from id in ids
join p in parents on id equals p.Id
select p
The result is parents 3, 7, 2, 4.
                So in a GroupJoin, the child values will contain objects, that contain the related values?
– duyn9uyen
                Mar 24, 2013 at 18:18
                As you said GroupJoin is like a outer join but that syntax (purely linq for group join) says it is not like outer join but left outer join.
– Imad
                Oct 20, 2015 at 10:29
  
The best way to get to grips with what GroupJoin does is to think of
  Join. There, the overall idea was that we looked through the "outer"
  input sequence, found all the matching items from the "inner" sequence
  (based on a key projection on each sequence) and then yielded pairs of
  matching elements. GroupJoin is similar, except that instead of
  yielding pairs of elements, it yields a single result for each "outer"
  item based on that item and the sequence of matching "inner" items.
The only difference is in return statement:
Join:
var lookup = inner.ToLookup(innerKeySelector, comparer); 
foreach (var outerElement in outer) 
    var key = outerKeySelector(outerElement); 
    foreach (var innerElement in lookup[key]) 
        yield return resultSelector(outerElement, innerElement); 
GroupJoin:
var lookup = inner.ToLookup(innerKeySelector, comparer); 
foreach (var outerElement in outer) 
    var key = outerKeySelector(outerElement); 
    yield return resultSelector(outerElement, lookup[key]); 
Read more here:
Reimplementing LINQ to Objects: Part 19 - Join
Reimplementing LINQ to Objects: Part 22 - GroupJoin
        new Person("Sudi", "sudi@try.cd"),
        new Person("Simba", "simba@try.cd"),
        new Person("Sarah", string.Empty)
    var records = new Data[]
        new Data("sudi@try.cd", "Sudi_Try"),
        new Data("sudi@try.cd", "Sudi@Test"),
        new Data("simba@try.cd", "SimbaLion")
You will note that sudi@try.cd has got two slackIds. I have made that for demonstrating how
Join works.
Let's now construct the query to join Person with Data:
var query = people.Join(records,
        x => x.Email,
        y => y.Mail,
        (person, record) => new { Name = person.Name, SlackId = record.SlackId});
    Console.WriteLine(query);
After constructing the query, you could also iterate over it with a foreach like so:
foreach (var item in query)
        Console.WriteLine($"{item.Name} has Slack ID {item.SlackId}");
Let's also output the result for GroupJoin:
Console.WriteLine(
        people.GroupJoin(
            records,
            x => x.Email,
            y => y.Mail,
            (person, recs) => new {
                Name = person.Name,
                SlackIds = recs.Select(r => r.SlackId).ToArray() // You could materialize //whatever way you want.
You will notice that the GroupJoin will put all SlackIds in a single group.
        
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