Linq to Entities join vs groupjoin

Question

I have web searched but I still cant find a simple answer  Can someone please explain  in simple English  what a GroupJoin is  How is it different from a regular inner Join  Is it commonly used  Is it only for method syntax  What about query syntax  A c  code example would be nice

User · Answer

According to eduLINQ      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

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Behaviour Suppose you have two lists   Id  Value 1   A 2   B 3   C  Id  ChildValue 1   a1 1   a2 1   a3 2   b1 2   b2  When you Join the two lists on the Id field the result will be  Value ChildValue A     a1 A     a2 A     a3 B     b1 B     b2  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 lt Parent gt  and IEnumerable lt Child gt  respectively   In case of Linq to Entities  IQueryable lt T gt    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 lt X gt  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 lt Y gt  where Y is an anonymous type consisting of one property of type Parent and a property of type IEnumerable lt Child gt   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 lt IEnumerable lt Child gt  gt   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     lt   into from c in g DefaultIfEmpty                     lt   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   gt  p Id  c   gt  c Id   p  c    gt  new   p  c            SelectMany x   gt  x c DefaultIfEmpty     x c    gt  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   gt  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

User · Answer

Let s suppose you have two different classes  public class Person       public string Name  Email           public Person string name  string email                Name   name          Email   email           class Data       public string Mail  SlackId           public Data string mail  string slackId                Mail   mail          SlackId   slackId           Now  let s Prepare data to work with  var people   new Person                 new Person  quot Sudi quot    quot sudi try cd quot            new Person  quot Simba quot    quot simba try cd quot            new Person  quot Sarah quot   string Empty                  var records   new Data                 new Data  quot sudi try cd quot    quot Sudi Try quot            new Data  quot sudi try cd quot    quot Sudi Test quot            new Data  quot simba try cd quot    quot SimbaLion quot           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   gt  x Email          y   gt  y Mail           person  record    gt  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   quot  item Name  has Slack ID  item SlackId  quot           Let s also output the result for GroupJoin  Console WriteLine               people GroupJoin              records              x   gt  x Email              y   gt  y Mail               person  recs    gt  new                   Name   person Name                  SlackIds   recs Select r   gt  r SlackId  ToArray      You could materialize   whatever way you want                             You will notice that the GroupJoin will put all SlackIds in a single group

[c#] Linq to Entities join vs groupjoin

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