How to do joins in LINQ on multiple fields in single join

Question

I need to do a LINQ2DataSet query that does a join on more than one field  as  var result   from x in entity join y in entity2         on x field1   y field1  and            x field2   y field2   I have yet found a suitable solution  I can add the extra constraints to a where clause  but this is far from a suitable solution  or use this solution  but that assumes an equijoin    Is it possible in LINQ to join on multiple fields in a single join   EDIT  var result   from x in entity              join y in entity2              on new   x field1  x field2   equals new   y field1  y field2     is the solution I referenced as assuming an equijoin above   Further EDIT  To answer criticism that my original example was an equijoin  I do acknowledge that  My current requirement is for an equijoin and I have already employed the solution I referenced above   I am  however  trying to understand what possibilities and best practices I have   should employ with LINQ  I am going to need to do a Date range query join with a table ID soon  and was just pre-empting that issue  It looks like I shall have to add the date range in the where clause   Thanks  as always  for all suggestions and comments given

User · Answer

As a full method chain that would look like this   lista SelectMany a   gt  listb Where xi   gt  b Id    a Id  amp  amp  b Total    a Total                    a  b    gt  new ResultItem                                       Id   a Id                      ATotal   a Total                      BTotal   b Total                    ToList

User · Answer

you could do something like  below    var query   from p in context T1          join q in context T2          on          new   p Col1  p Col2            equals           new   q Col1  q Col2            select new  p      q

User · Answer

If the field name are different in entities   var result   from x in entity    join y in entity2 on            new                   field1    x field1                 field2    x field2                            equals          new                    field1  y field1                  field2   y myfield                 select new  x y

User · Answer

var result   from x in entity    join y in entity2 on new   x field1  x field2   equals new   y field1  y field2

User · Answer

I think a more readable and flexible option is to use Where function   var result   from x in entity1              from y in entity2                   Where y   gt  y field1    x field1  amp  amp  y field2    x field2    This also allows to easily change from inner join to left join by appending  DefaultIfEmpty

User · Answer

Declare a Class Type  to hold the elements you want to join  In the below example declare JoinElement   public class   JoinElement         public int  Id   get  set        public string Name   get  set        results   from course in courseQueryable AsQueryable                     join agency in agencyQueryable AsQueryable                      on new   JoinElement       Id   course CourseAgencyId  Name   course CourseDeveloper                       equals new   JoinElement       Id   agency CourseAgencyId  Name    D    into temp1

User · Answer

I think a more readable and flexible option is to use Where function   var result   from x in entity1              from y in entity2                   Where y   gt  y field1    x field1  amp  amp  y field2    x field2    This also allows to easily change from inner join to left join by appending  DefaultIfEmpty

User · Answer

var result   from x in entity    join y in entity2 on new   x field1  x field2   equals new   y field1  y field2

User · Answer

var result   from x in entity1              join y in entity2              on new   X1  x field1  X2  x field2   equals new   X1 y field1  X2  y field2     You need to do this  if the column names are different in two entities

User · Answer

The solution with the anonymous type should work fine  LINQ can only represent equijoins  with join clauses  anyway   and indeed that s what you ve said you want to express anyway based on your original query   If you don t like the version with the anonymous type for some specific reason  you should explain that reason   If you want to do something other than what you originally asked for  please give an example of what you really want to do   EDIT  Responding to the edit in the question  yes  to do a  date range  join  you need to use a where clause instead  They re semantically equivalent really  so it s just a matter of the optimisations available  Equijoins provide simple optimisation  in LINQ to Objects  which includes LINQ to DataSets  by creating a lookup based on the inner sequence - think of it as a hashtable from key to a sequence of entries matching that key   Doing that with date ranges is somewhat harder  However  depending on exactly what you mean by a  date range join  you may be able to do something similar - if you re planning on creating  bands  of dates  e g  one per year  such that two entries which occur in the same year  but not on the same date  should match  then you can do it just by using that band as the key  If it s more complicated  e g  one side of the join provides a range  and the other side of the join provides a single date  matching if it falls within that range  that would be better handled with a where clause  after a second from clause  IMO  You could do some particularly funky magic by ordering one side or the other to find matches more efficiently  but that would be a lot of work - I d only do that kind of thing after checking whether performance is an issue

User · Answer

you could do something like  below    var query   from p in context T1          join q in context T2          on          new   p Col1  p Col2            equals           new   q Col1  q Col2            select new  p      q

User · Answer

I used tuples to do that  this is an example for two columns    var list  list1 Join list2                         e1   gt   e1 val1 e1 val2                          e2   gt   e2 val1 e2 val2                           e1  e2    gt  e1  ToList

User · Answer

var result   from x in entity              join y in entity2              on new   X1  x field1  X2  x field2   equals new   X1 y field1  X2  y field2                select new                                    Columns

User · Answer

I used tuples to do that  this is an example for two columns    var list  list1 Join list2                         e1   gt   e1 val1 e1 val2                          e2   gt   e2 val1 e2 val2                           e1  e2    gt  e1  ToList

User · Answer

you could do something like  below    var query   from p in context T1          join q in context T2          on          new   p Col1  p Col2            equals           new   q Col1  q Col2            select new  p      q

User · Answer

The solution with the anonymous type should work fine  LINQ can only represent equijoins  with join clauses  anyway   and indeed that s what you ve said you want to express anyway based on your original query   If you don t like the version with the anonymous type for some specific reason  you should explain that reason   If you want to do something other than what you originally asked for  please give an example of what you really want to do   EDIT  Responding to the edit in the question  yes  to do a  date range  join  you need to use a where clause instead  They re semantically equivalent really  so it s just a matter of the optimisations available  Equijoins provide simple optimisation  in LINQ to Objects  which includes LINQ to DataSets  by creating a lookup based on the inner sequence - think of it as a hashtable from key to a sequence of entries matching that key   Doing that with date ranges is somewhat harder  However  depending on exactly what you mean by a  date range join  you may be able to do something similar - if you re planning on creating  bands  of dates  e g  one per year  such that two entries which occur in the same year  but not on the same date  should match  then you can do it just by using that band as the key  If it s more complicated  e g  one side of the join provides a range  and the other side of the join provides a single date  matching if it falls within that range  that would be better handled with a where clause  after a second from clause  IMO  You could do some particularly funky magic by ordering one side or the other to find matches more efficiently  but that would be a lot of work - I d only do that kind of thing after checking whether performance is an issue

User · Answer

Using the join operator you can only perform equijoins   Other types of joins can be constructed using other operators   I m not sure whether the exact join you are trying to do would be easier using these methods or by changing the where clause   Documentation on the join clause can be found here   MSDN has an article on join operations with multiple links to examples of other joins  as well

User · Answer

Just to complete this with an equivalent method chain syntax   entity Join entity2  x   gt  new  x Field1  x Field2                        y   gt  new  y Field1  y Field2    x  y    gt  x     While the last argument  x  y    gt  x is what you select  in the above case we select x

User · Answer

Declare a Class Type  to hold the elements you want to join  In the below example declare JoinElement   public class   JoinElement         public int  Id   get  set        public string Name   get  set        results   from course in courseQueryable AsQueryable                     join agency in agencyQueryable AsQueryable                      on new   JoinElement       Id   course CourseAgencyId  Name   course CourseDeveloper                       equals new   JoinElement       Id   agency CourseAgencyId  Name    D    into temp1

User · Answer

Using the join operator you can only perform equijoins   Other types of joins can be constructed using other operators   I m not sure whether the exact join you are trying to do would be easier using these methods or by changing the where clause   Documentation on the join clause can be found here   MSDN has an article on join operations with multiple links to examples of other joins  as well

User · Answer

Using the join operator you can only perform equijoins   Other types of joins can be constructed using other operators   I m not sure whether the exact join you are trying to do would be easier using these methods or by changing the where clause   Documentation on the join clause can be found here   MSDN has an article on join operations with multiple links to examples of other joins  as well

User · Answer

var result   from x in entity    join y in entity2 on new   x field1  x field2   equals new   y field1  y field2

User · Answer

Just to complete this with an equivalent method chain syntax   entity Join entity2  x   gt  new  x Field1  x Field2                        y   gt  new  y Field1  y Field2    x  y    gt  x     While the last argument  x  y    gt  x is what you select  in the above case we select x

User · Answer

As a full method chain that would look like this   lista SelectMany a   gt  listb Where xi   gt  b Id    a Id  amp  amp  b Total    a Total                    a  b    gt  new ResultItem                                       Id   a Id                      ATotal   a Total                      BTotal   b Total                    ToList

User · Answer

var result   from x in entity1              join y in entity2              on new   X1  x field1  X2  x field2   equals new   X1 y field1  X2  y field2     You need to do this  if the column names are different in two entities

User · Answer

var result   from x in entity    join y in entity2 on new   x field1  x field2   equals new   y field1  y field2

User · Answer

from d in db CourseDispatches                              join du in db DispatchUsers on d id equals du dispatch id                              join u in db Users on du user id equals u id                              join fr in db Forumreports on  d course id         du user id   equals   fr course id        fr uid    this works for me

User · Answer

If the field name are different in entities   var result   from x in entity    join y in entity2 on            new                   field1    x field1                 field2    x field2                            equals          new                    field1  y field1                  field2   y myfield                 select new  x y

User · Answer

The solution with the anonymous type should work fine  LINQ can only represent equijoins  with join clauses  anyway   and indeed that s what you ve said you want to express anyway based on your original query   If you don t like the version with the anonymous type for some specific reason  you should explain that reason   If you want to do something other than what you originally asked for  please give an example of what you really want to do   EDIT  Responding to the edit in the question  yes  to do a  date range  join  you need to use a where clause instead  They re semantically equivalent really  so it s just a matter of the optimisations available  Equijoins provide simple optimisation  in LINQ to Objects  which includes LINQ to DataSets  by creating a lookup based on the inner sequence - think of it as a hashtable from key to a sequence of entries matching that key   Doing that with date ranges is somewhat harder  However  depending on exactly what you mean by a  date range join  you may be able to do something similar - if you re planning on creating  bands  of dates  e g  one per year  such that two entries which occur in the same year  but not on the same date  should match  then you can do it just by using that band as the key  If it s more complicated  e g  one side of the join provides a range  and the other side of the join provides a single date  matching if it falls within that range  that would be better handled with a where clause  after a second from clause  IMO  You could do some particularly funky magic by ordering one side or the other to find matches more efficiently  but that would be a lot of work - I d only do that kind of thing after checking whether performance is an issue

User · Answer

you could do something like  below    var query   from p in context T1          join q in context T2          on          new   p Col1  p Col2            equals           new   q Col1  q Col2            select new  p      q

User · Answer

from d in db CourseDispatches                              join du in db DispatchUsers on d id equals du dispatch id                              join u in db Users on du user id equals u id                              join fr in db Forumreports on  d course id         du user id   equals   fr course id        fr uid    this works for me

User · Answer

var result   from x in entity              join y in entity2              on new   X1  x field1  X2  x field2   equals new   X1 y field1  X2  y field2                select new                                    Columns

User · Answer

Using the join operator you can only perform equijoins   Other types of joins can be constructed using other operators   I m not sure whether the exact join you are trying to do would be easier using these methods or by changing the where clause   Documentation on the join clause can be found here   MSDN has an article on join operations with multiple links to examples of other joins  as well

User · Answer

The solution with the anonymous type should work fine  LINQ can only represent equijoins  with join clauses  anyway   and indeed that s what you ve said you want to express anyway based on your original query   If you don t like the version with the anonymous type for some specific reason  you should explain that reason   If you want to do something other than what you originally asked for  please give an example of what you really want to do   EDIT  Responding to the edit in the question  yes  to do a  date range  join  you need to use a where clause instead  They re semantically equivalent really  so it s just a matter of the optimisations available  Equijoins provide simple optimisation  in LINQ to Objects  which includes LINQ to DataSets  by creating a lookup based on the inner sequence - think of it as a hashtable from key to a sequence of entries matching that key   Doing that with date ranges is somewhat harder  However  depending on exactly what you mean by a  date range join  you may be able to do something similar - if you re planning on creating  bands  of dates  e g  one per year  such that two entries which occur in the same year  but not on the same date  should match  then you can do it just by using that band as the key  If it s more complicated  e g  one side of the join provides a range  and the other side of the join provides a single date  matching if it falls within that range  that would be better handled with a where clause  after a second from clause  IMO  You could do some particularly funky magic by ordering one side or the other to find matches more efficiently  but that would be a lot of work - I d only do that kind of thing after checking whether performance is an issue

[c#] How to do joins in LINQ on multiple fields in single join

Examples related to c#

Examples related to linq

Examples related to join