Difference Between Select and SelectMany

Question

I ve been searching the difference between Select and SelectMany but I haven t been able to find a suitable answer  I need to learn the difference when using LINQ To SQL but all I ve found are standard array examples    Can someone provide a LINQ To SQL example

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var players   db SoccerTeams Where c   gt  c Country     Spain                                SelectMany c   gt  c players    foreach var player in players        Console WriteLine player LastName        De Gea  Alba Costa Villa Busquets

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It s more clear when the query return a string  an array of char    For example if the list  Fruits  contains  apple    Select  returns the string   Fruits Select s  gt s     0    apple     SelectMany  flattens the string    Fruits SelectMany s  gt s    0   97   a   1   112  p   2   112  p   3   108  l   4   101  e

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Some SelectMany may not be necessary  Below 2 queries give the same result    Customers Where c  gt c Name   Tom   SelectMany c  gt c Orders   Orders Where o  gt o Customer Name   Tom     For 1-to-Many relationship     if Start from  1   SelectMany is needed  it flattens the many   if Start from  Many   SelectMany is not needed   still be able to filter from  1   also this is simpler than below standard join query      from o in Orders join c in Customers on o CustomerID equals c ID where c Name     Tom  select o

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Here is a code example with an initialized small collection for testing   class Program       static void Main string   args                List lt Order gt  orders   new List lt Order gt                        new Order                               OrderID    orderID1                   OrderLines   new List lt OrderLine gt                                        new OrderLine                                               ProductSKU    SKU1                           Quantity   1                                            new OrderLine                                               ProductSKU    SKU2                           Quantity   2                                            new OrderLine                                               ProductSKU    SKU3                           Quantity   3                                                                    new Order                               OrderID    orderID2                   OrderLines   new List lt OrderLine gt                                        new OrderLine                                               ProductSKU    SKU4                           Quantity   4                                            new OrderLine                                               ProductSKU    SKU5                           Quantity   5                                                                             required result is the list of all SKUs in orders         List lt string gt  allSKUs   new List lt string gt                With Select case 2 foreach loops are required         var flattenedOrdersLinesSelectCase   orders Select o   gt  o OrderLines           foreach  var flattenedOrderLine in flattenedOrdersLinesSelectCase                        foreach  OrderLine orderLine in flattenedOrderLine                                allSKUs Add orderLine ProductSKU                                      With SelectMany case only one foreach loop is required         allSKUs   new List lt string gt             var flattenedOrdersLinesSelectManyCase   orders SelectMany o   gt  o OrderLines           foreach  var flattenedOrderLine in flattenedOrdersLinesSelectManyCase                        allSKUs Add flattenedOrderLine ProductSKU                       If the required result is flattened list which has OrderID  ProductSKU and Quantity           SelectMany with selector is very helpful to get the required result          and allows avoiding own For loops what according to my experience do code faster when           hundreds of thousands of data rows must be operated         List lt OrderLineForReport gt  ordersLinesForReport    List lt OrderLineForReport gt  orders SelectMany o   gt  o OrderLines               o  ol    gt  new OrderLineForReport                               OrderID   o OrderID                  ProductSKU   ol ProductSKU                  Quantity   ol Quantity                ToList            class Order       public string OrderID   get  set        public List lt OrderLine gt  OrderLines   get  set      class OrderLine       public string ProductSKU   get  set        public int Quantity   get  set      class OrderLineForReport       public string OrderID   get  set        public string ProductSKU   get  set        public int Quantity   get  set

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It is the best way to understand i think               var query               Enumerable                  Range 1  10                   SelectMany ints   gt  Enumerable Range 1  10    a  b    gt     a     b     a   b                     ToArray             Console WriteLine string Join Environment NewLine  query             Console Read      Multiplication Table example

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The SelectMany method knocks down an IEnumerable lt IEnumerable lt T gt  gt  into an IEnumerable lt T gt   like communism  every element is behaved in the same manner a stupid guy has same rights of a genious one    var words   new       a b c    d e    f     var splitAndCombine   words SelectMany x   gt  x Split           returns    a    b    c    d    e    f

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Select many is like cross join operation in SQL where it takes the cross product  For example if we have  Set A  a b c  Set B  x y    Select many can be used to get the following set     x a     x b     x c     y a     y b     y c      Note that here we take the all the possible combinations that can be made from the elements of set A and set B    Here is a LINQ example you can try  List lt string gt  animals   new List lt string gt       cat    dog    donkey     List lt int gt  number   new List lt int gt      10  20     var mix   number SelectMany num   gt  animals   n  a    gt  new   n  a       the mix will have following elements in flat structure like      10 cat    10 dog    10 donkey    20 cat    20 dog    20 donkey

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Consider this example            var array   new string 2                         I like what I like                I like what you like                       query1 returns two elements sth like this            fisrt element would be array 5     0     I   like   what   I   like            second element would be array 5    1     I   like   what   you   like          IEnumerable lt string   gt  query1   array Select s   gt  s Split       Distinct               query2 return back flat result sth like this               I   like   what   you          IEnumerable lt string gt  query2   array SelectMany s   gt  s Split       Distinct      So as you see duplicate values like  I  or  like  have been removed from query2 because  SelectMany  flattens and projects across multiple sequences  But query1 returns sequence of string arrays  and since there are two different arrays in query1  first and second element   nothing would be removed

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There are several overloads to SelectMany  One of them allows you to keep trace of any relationship between parent and children while traversing the hierarchy   Example  suppose you have the following structure  League - gt  Teams - gt  Player   You can easily return a flat collection of players  However you may lose any reference to the team the player is part of   Fortunately there is an overload for such purpose   var teamsAndTheirLeagues             from helper in leagues SelectMany                  l   gt  l Teams                      league  team     gt  new   league  team                           where helper team Players Count  gt  2                              amp  amp  helper league Teams Count  lt  10                            select new                                      LeagueID   helper league ID                                       Team   helper team                                          The previous example is taken from Dan s IK blog  I strongly recommend you take a look at it

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Select is a simple one-to-one projection from source element to a result element  Select- Many is used when there are multiple from clauses in a query expression  each element in the original sequence is used to generate a new sequence

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One more example how SelectMany   Select can be used in order to accumulate sub array objects data   Suppose we have users with they phones   class Phone        public string BasePart    555-xxx-xxx       class User        public string Name    Xxxxx       public List lt Phone gt  Phones       Now we need to select all phones  BaseParts of all users   var usersArray   new List lt User gt        array of arrays List lt string gt  allBaseParts   usersArray SelectMany ua   gt  ua Phones  Select p   gt  p BasePart  ToList

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I understand SelectMany to work like a join shortcut   So you can   var orders   customers               Where c   gt  c CustomerName     Acme                 SelectMany c   gt  c Orders

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The SelectMany   method is used to flatten a sequence in which each of the elements of the sequence is a separate   I have class user same like this  class User               public string UserName   get  set            public List lt string gt  Roles   get  set            main   var users   new List lt User gt                                new User   UserName    Reza    Roles   new List lt string gt   Superadmin                       new User   UserName    Amin    Roles   new List lt string gt   Guest   Reseption                       new User   UserName    Nima    Roles   new List lt string gt   Nurse   Guest                       var query   users SelectMany user   gt  user Roles   user  role    gt  new   user UserName  role      foreach  var obj in query        Console WriteLine obj       output      UserName   Reza  role   Superadmin       UserName   Amin  role   Guest       UserName   Amin  role   Reseption       UserName   Nima  role   Nurse       UserName   Nima  role   Guest      You can use operations on any item of sequence   int     numbers                     new    1  2  3                   new    4                   new    5  6   6   2   7  8                   new    12  14                  IEnumerable lt int gt  result   numbers                  SelectMany array   gt  array Distinct                     OrderBy x   gt  x      output      1  2   2   3  4  5  6  7  8  12  14      List lt List lt int gt  gt  numbers   new List lt List lt int gt  gt                    new List lt int gt   1  2  3                   new List lt int gt   12                   new List lt int gt   5  6  5  7                   new List lt int gt   10  10  10  12                   IEnumerable lt int gt  result   numbers                  SelectMany list   gt  list                   Distinct                    OrderBy x  gt x      output       1  2  3  5  6  7  10  12

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Just for an alternate view that may help some functional programmers out there    Select is map SelectMany is bind  or flatMap for your Scala Kotlin people

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Without getting too technical - database with many Organizations  each with many Users -  var orgId    123456789    var userList1   db Organizations                     Where a   gt  a OrganizationId    orgId                      SelectMany a   gt  a Users                      ToList     var userList2   db Users                     Where a   gt  a OrganizationId    orgId                      ToList      both return the same ApplicationUser list for the selected Organization   The first  projects  from Organization to Users  the second queries the Users table directly

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SelectMany flattens queries that return lists of lists  For example  public class PhoneNumber       public string Number   get  set       public class Person       public IEnumerable lt PhoneNumber gt  PhoneNumbers   get  set        public string Name   get  set       IEnumerable lt Person gt  people   new List lt Person gt         Select gets a list of lists of phone numbers IEnumerable lt IEnumerable lt PhoneNumber gt  gt  phoneLists   people Select p   gt  p PhoneNumbers       SelectMany flattens it to just a list of phone numbers  IEnumerable lt PhoneNumber gt  phoneNumbers   people SelectMany p   gt  p PhoneNumbers       And to include data from the parent in the result      pass an expression to the second parameter  resultSelector  in the overload  var directory   people     SelectMany p   gt  p PhoneNumbers                  parent  child    gt  new   parent Name  child Number       Live Demo on  NET Fiddle

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SelectMany   lets you collapse a multidimensional sequence in a way that would otherwise require a second Select   or loop   More details at this blog post

[c#] Difference Between Select and SelectMany

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