Reputation: 6763
LEFT OUTER JOIN in LINQ
How to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses? Is there any way to do that with where clause?
Correct problem:
For inner join is easy and I have a solution like this
List<JoinPair> innerFinal = (from l in lefts from r in rights where l.Key == r.Key
select new JoinPair { LeftId = l.Id, RightId = r.Id})
but for left outer join I need a solution. Mine is something like this but it's not working
List< JoinPair> leftFinal = (from l in lefts from r in rights
select new JoinPair {
LeftId = l.Id,
RightId = ((l.Key==r.Key) ? r.Id : 0
})
where JoinPair is a class:
public class JoinPair { long leftId; long rightId; }
Upvotes: 671
Views: 1085063
Answers (24)
Reputation: 82186
If a database driven LINQ provider is used, a significantly more readable left outer join can be written as such:
from c in categories
from p in products.Where(c == p.Category).DefaultIfEmpty()
If you omit the DefaultIfEmpty() you will have an inner join.
Take the accepted answer:
from c in categories
join p in products on c equals p.Category into ps
from p in ps.DefaultIfEmpty()
This syntax is very confusing, and it's not clear how it works when you want to left join MULTIPLE tables.
Note
It should be noted that from alias in Repo.SomeTable.Where(condition).DefaultIfEmpty() is the same as an outer-apply/left-join-lateral, which any (decent) database-optimizer is perfectly capable of translating into a left join, as long as you don't introduce per-row-values (aka an actual outer apply). Don't do this in Linq-2-Objects, because there, there will be no DB-optimizer ...
Detailed Example
var query2 = (
from users in Repo.T_User
from mappings in Repo.T_User_Group
.Where(mapping => mapping.USRGRP_USR == users.USR_ID)
.DefaultIfEmpty() // <== makes join left join
from groups in Repo.T_Group
.Where(gruppe => gruppe.GRP_ID == mappings.USRGRP_GRP)
.DefaultIfEmpty() // <== makes join left join
// where users.USR_Name.Contains(keyword)
// || mappings.USRGRP_USR.Equals(666)
// || mappings.USRGRP_USR == 666
// || groups.Name.Contains(keyword)
select new
{
UserId = users.USR_ID
,UserName = users.USR_User
,UserGroupId = groups.ID
,GroupName = groups.Name
}
);
var xy = (query2).ToList();
When used with LINQ 2 SQL it will translate nicely to the following very legible SQL query:
SELECT
users.USR_ID AS UserId
,users.USR_User AS UserName
,groups.ID AS UserGroupId
,groups.Name AS GroupName
FROM T_User AS users
LEFT JOIN T_User_Group AS mappings
ON mappings.USRGRP_USR = users.USR_ID
LEFT JOIN T_Group AS groups
ON groups.GRP_ID == mappings.USRGRP_GRP
I stress again, if you are doing that in Linq-2-Objects (instead of Linq-2-SQL), you should do it the old-fashioned way (because LINQ to SQL translates this correctly to join operations, but over objects this method forces a full scan).
Here's the old fashioned way to do it, with actual left-joins:
var query2 = (
from users in Repo.T_Benutzer
join mappings in Repo.T_Benutzer_Benutzergruppen on mappings.BEBG_BE equals users.BE_ID into tmpMapp
join groups in Repo.T_Benutzergruppen on groups.ID equals mappings.BEBG_BG into tmpGroups
from mappings in tmpMapp.DefaultIfEmpty()
from groups in tmpGroups.DefaultIfEmpty()
select new
{
UserId = users.BE_ID
,UserName = users.BE_User
,UserGroupId = mappings.BEBG_BG
,GroupName = groups.Name
}
);
Now, as an example with a more complex query, if you need further explanation on how it works, consider this SQL statement:
DECLARE @BE_ID integer;
DECLARE @stichtag datetime;
DECLARE @in_standort uniqueidentifier;
DECLARE @in_gebaeude uniqueidentifier;
SET @BE_ID = 123;
SET @stichtag = CURRENT_TIMESTAMP;
SET @in_standort = '00000000-0000-0000-0000-000000000000';
SET @in_gebaeude = '00000000-0000-0000-0000-000000000000';
DECLARE @unixTimestamp bigint;
DECLARE @bl national character varying(MAX);
SET @unixTimestamp = DATEDIFF(SECOND, '1970-01-01T00:00:00.000', CONVERT(DATETIME, @stichtag, 1));
SET @bl = (
SELECT TOP 1 FC_Value
FROM T_FMS_Configuration
WHERE FC_Key = 'basicLink'
);
-- SELECT @unixTimestamp AS unix_ts, @bl AS bl;
SELECT
so.SO_Nr AS RPT_SO_Nr
,so.SO_Bezeichnung AS RPT_SO_Bezeichnung
,gb.GB_Bezeichnung
,gb.GB_GM_Lat
,gb.GB_GM_Lng
,objTyp.OBJT_Code
,@bl + '/Modules/App150/index.html'
+ '?Code=' + COALESCE(objTyp.OBJT_Code, 'BAD')
+ '&UID=' + COALESCE(CAST(gb.GB_UID AS national character varying(MAX)), '')
+ '&Timestamp=' + CONVERT(national character varying(MAX), @unixTimestamp, 126)
AS RPT_QR
FROM T_AP_Gebaeude AS gb
LEFT JOIN T_AP_Standort AS so ON gb.GB_SO_UID = so.SO_UID
LEFT JOIN T_OV_Ref_ObjektTyp AS objTyp ON 'GB' = objTyp.OBJT_Code
LEFT JOIN T_Benutzer AS benutzer ON benutzer.BE_ID = @BE_ID AND benutzer.BE_Status = 1
WHERE gb.GB_Status = 1
AND @stichtag >= gb.GB_DatumVon
AND @stichtag <= gb.GB_DatumBis
AND so.SO_Status = 1
AND @stichtag >= so.SO_DatumVon
AND @stichtag <= so.SO_DatumBis
AND (@in_standort = '00000000-0000-0000-0000-000000000000' OR so.SO_UID = @in_standort)
AND (@in_gebaeude = '00000000-0000-0000-0000-000000000000' OR gb.GB_UID = @in_gebaeude)
AND
(
benutzer.BE_ID IS NULL
OR benutzer.BE_ID < 0
OR benutzer.BE_usePRT = 0
OR EXISTS
(
SELECT 1
FROM T_COR_Objekte AS obj
INNER JOIN T_COR_ZO_ObjektRechte_Lesen AS objR
ON objR.ZO_OBJR_OBJ_UID = obj.OBJ_UID
AND objR.ZO_OBJR_OBJ_OBJT_Code = obj.OBJ_OBJT_Code
WHERE obj.OBJ_UID = gb.GB_UID
)
);
which yields the following LINQ:
(this is the DB context, dump is a method of LINQpad)
int BE_ID = 123;
System.DateTime stichtag = System.DateTime.Now;
System.Guid in_standort = System.Guid.Empty;
System.Guid in_gebaeude = System.Guid.Empty;
long unixTimestamp = (long)(stichtag.ToUniversalTime() - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalSeconds;
string bl = (
from c in this.T_FMS_Configuration
where c.FC_Key == "basicLink"
select c.FC_Value
).FirstOrDefault();
(
from gb in this.T_AP_Gebaeude
join so in this.T_AP_Standort on gb.GB_SO_UID equals so.SO_UID into gb_so
from so in gb_so.DefaultIfEmpty()
join objTyp in this.T_OV_Ref_ObjektTyp on "GB" equals objTyp.OBJT_Code into gb_objTyp
from objTyp in gb_objTyp.DefaultIfEmpty()
join benutzer in this.T_Benutzer.Where(b => b.BE_ID == BE_ID && b.BE_Status == 1) on 1 equals 1 into gb_benutzer
from benutzer in gb_benutzer.DefaultIfEmpty()
where gb.GB_Status == 1
&& stichtag >= gb.GB_DatumVon
&& stichtag <= gb.GB_DatumBis
&& so.SO_Status == 1
&& stichtag >= so.SO_DatumVon
&& stichtag <= so.SO_DatumBis
&& (in_standort == System.Guid.Empty|| so.SO_UID == in_standort)
&& (in_gebaeude == System.Guid.Empty || gb.GB_UID == in_gebaeude)
&&
(
benutzer == null
|| benutzer.BE_ID < 0
|| benutzer.BE_usePRT == false
|| this.T_COR_Objekte.Any(
obj => obj.OBJ_UID == gb.GB_UID
&& this.T_COR_ZO_ObjektRechte_Lesen.Any(objR => objR.ZO_OBJR_OBJ_UID == obj.OBJ_UID && objR.ZO_OBJR_OBJ_OBJT_Code == obj.OBJ_OBJT_Code)
)
)
select new {
RPT_SO_Nr = so.SO_Nr
,RPT_SO_Bezeichnung = so.SO_Bezeichnung
// ,RPT_GB_UID = gb.GB_UID
// ,gb.GB_Nr
,gb.GB_Bezeichnung
// ,adr = gb.GB_Strasse + " " + gb.GB_StrasseNr + ", CH-" + gb.GB_PLZ + " " + gb.GB_Ort
,gb.GB_GM_Lat
,gb.GB_GM_Lng
// ,objTyp.OBJT_UID
,objTyp.OBJT_Code
,RPT_QR = bl + "/Modules/App150/index.html"
+ "?Code=" + (objTyp.OBJT_Code ?? "BAD")
+ "&UID=" + (System.Convert.ToString(gb.GB_UID) ?? "" )
+ "&Timestamp=" + unixTimestamp.ToString(System.Globalization.CultureInfo.InvariantCulture)
}
).Dump();
Upvotes: 661
Reputation: 61401
Now as an extension method:
public static class LinqExt
{
public static IEnumerable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TKey> leftKey, Func<TRight, TKey> rightKey,
Func<TLeft, TRight, TResult> result)
{
return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
.SelectMany(
o => o.r.DefaultIfEmpty(),
(l, r) => new { lft= l.l, rght = r })
.Select(o => result.Invoke(o.lft, o.rght));
}
}
Use like you would normally use join:
var contents = list.LeftOuterJoin(list2,
l => l.country,
r => r.name,
(l, r) => new { count = l.Count(), l.country, l.reason, r.people })
Hope this saves you some time.
Upvotes: 62
Reputation: 926
This is the LeftJoin implementation I use. Notice that the the resultSelector expression accepts 2 parameters: one instance from both sides of the join. In most other implementations that I've seen the result selector only accepts one parameter, which is a "join model" with a left/right or outer/inner property. I like this implementation better because it has the same method signature as the built-in Join method. It also works with IQueryables and EF.
var results = DbContext.Categories
.LeftJoin(
DbContext.Products, c => c.Id, p => p.CategoryId,
(c, p) => new { Category = c, ProductName = p == null ? "(No Products)" : p.ProductName })
.ToList();
public static class QueryableExtensions
{
public static IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IEnumerable<TInner> inner, Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
var query = outer
.GroupJoin(inner, outerKeySelector, innerKeySelector, (o, i) => new { o, i })
.SelectMany(o => o.i.DefaultIfEmpty(), (x, i) => new { x.o, i });
return ApplySelector(query, x => x.o, x => x.i, resultSelector);
}
private static IQueryable<TResult> ApplySelector<TSource, TOuter, TInner, TResult>(
IQueryable<TSource> source,
Expression<Func<TSource, TOuter>> outerProperty,
Expression<Func<TSource, TInner>> innerProperty,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
var p = Expression.Parameter(typeof(TSource), $"param_{Guid.NewGuid()}".Replace("-", string.Empty));
Expression body = resultSelector?.Body
.ReplaceParameter(resultSelector.Parameters[0], outerProperty.Body.ReplaceParameter(outerProperty.Parameters[0], p))
.ReplaceParameter(resultSelector.Parameters[1], innerProperty.Body.ReplaceParameter(innerProperty.Parameters[0], p));
var selector = Expression.Lambda<Func<TSource, TResult>>(body, p);
return source.Select(selector);
}
}
public static class ExpressionExtensions
{
public static Expression ReplaceParameter(this Expression source, ParameterExpression toReplace, Expression newExpression)
=> new ReplaceParameterExpressionVisitor(toReplace, newExpression).Visit(source);
}
public class ReplaceParameterExpressionVisitor : ExpressionVisitor
{
public ReplaceParameterExpressionVisitor(ParameterExpression toReplace, Expression replacement)
{
this.ToReplace = toReplace;
this.Replacement = replacement;
}
public ParameterExpression ToReplace { get; }
public Expression Replacement { get; }
protected override Expression VisitParameter(ParameterExpression node)
=> (node == ToReplace) ? Replacement : base.VisitParameter(node);
}
Upvotes: 2
Reputation: 103
This is the prettiest solution I use, give it a try! 😉
(from c in categories
let product = products.Where(d=> d.Category == c.Category).FirstOrDefault()
select new { Category = c, ProductName = p == null ? "(No products)" : product.ProductName };
Upvotes: 0
Reputation: 17931
As stated in "Perform left outer joins":
var q =
from c in categories
join pt in products on c.Category equals pt.Category into ps_jointable
from p in ps_jointable.DefaultIfEmpty()
select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };
Upvotes: 754
Reputation: 61
Here's a version of the extension method solution using IQueryable instead of IEnumerable
public class OuterJoinResult<TLeft, TRight>
{
public TLeft LeftValue { get; set; }
public TRight RightValue { get; set; }
}
public static IQueryable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IQueryable<TLeft> left, IQueryable<TRight> right, Expression<Func<TLeft, TKey>> leftKey, Expression<Func<TRight, TKey>> rightKey, Expression<Func<OuterJoinResult<TLeft, TRight>, TResult>> result)
{
return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
.SelectMany(o => o.r.DefaultIfEmpty(), (l, r) => new OuterJoinResult<TLeft, TRight> { LeftValue = l.l, RightValue = r })
.Select(result);
}
Upvotes: 2
Reputation: 4243
Overview: In this code snippet, I demonstrate how to group by ID where Table1 and Table2 have a one to many relationship. I group on Id, Field1, and Field2. The subquery is helpful, if a third Table lookup is required and it would have required a left join relationship. I show a left join grouping and a subquery linq. The results are equivalent.
class MyView
{
public integer Id {get,set};
public String Field1 {get;set;}
public String Field2 {get;set;}
public String SubQueryName {get;set;}
}
IList<MyView> list = await (from ci in _dbContext.Table1
join cii in _dbContext.Table2
on ci.Id equals cii.Id
where ci.Field1 == criterion
group new
{
ci.Id
} by new { ci.Id, cii.Field1, ci.Field2}
into pg
select new MyView
{
Id = pg.Key.Id,
Field1 = pg.Key.Field1,
Field2 = pg.Key.Field2,
SubQueryName=
(from chv in _dbContext.Table3 where chv.Id==pg.Key.Id select chv.Field1).FirstOrDefault()
}).ToListAsync<MyView>();
Compared to using a Left Join and Group new
IList<MyView> list = await (from ci in _dbContext.Table1
join cii in _dbContext.Table2
on ci.Id equals cii.Id
join chv in _dbContext.Table3
on cii.Id equals chv.Id into lf_chv
from chv in lf_chv.DefaultIfEmpty()
where ci.Field1 == criterion
group new
{
ci.Id
} by new { ci.Id, cii.Field1, ci.Field2, chv.FieldValue}
into pg
select new MyView
{
Id = pg.Key.Id,
Field1 = pg.Key.Field1,
Field2 = pg.Key.Field2,
SubQueryName=pg.Key.FieldValue
}).ToListAsync<MyView>();
Upvotes: 0
Reputation: 3035
Using lambda expression
db.Categories
.GroupJoin(db.Products,
Category => Category.CategoryId,
Product => Product.CategoryId,
(x, y) => new { Category = x, Products = y })
.SelectMany(
xy => xy.Products.DefaultIfEmpty(),
(x, y) => new { Category = x.Category, Product = y })
.Select(s => new
{
CategoryName = s.Category.Name,
ProductName = s.Product.Name
});
Upvotes: 190
Reputation: 3661
As per my answer to a similar question, here:
Linq to SQL left outer join using Lambda syntax and joining on 2 columns (composite join key)
Get the code here, or clone my github repo, and play!
Query:
var petOwners =
from person in People
join pet in Pets
on new
{
person.Id,
person.Age,
}
equals new
{
pet.Id,
Age = pet.Age * 2, // owner is twice age of pet
}
into pets
from pet in pets.DefaultIfEmpty()
select new PetOwner
{
Person = person,
Pet = pet,
};
Lambda:
var petOwners = People.GroupJoin(
Pets,
person => new { person.Id, person.Age },
pet => new { pet.Id, Age = pet.Age * 2 },
(person, pet) => new
{
Person = person,
Pets = pet,
}).SelectMany(
pet => pet.Pets.DefaultIfEmpty(),
(people, pet) => new
{
people.Person,
Pet = pet,
});
Upvotes: 3
Reputation: 15327
take look at this example
class Person
{
public int ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string Phone { get; set; }
}
class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
}
public static void LeftOuterJoinExample()
{
Person magnus = new Person {ID = 1, FirstName = "Magnus", LastName = "Hedlund"};
Person terry = new Person {ID = 2, FirstName = "Terry", LastName = "Adams"};
Person charlotte = new Person {ID = 3, FirstName = "Charlotte", LastName = "Weiss"};
Person arlene = new Person {ID = 4, FirstName = "Arlene", LastName = "Huff"};
Pet barley = new Pet {Name = "Barley", Owner = terry};
Pet boots = new Pet {Name = "Boots", Owner = terry};
Pet whiskers = new Pet {Name = "Whiskers", Owner = charlotte};
Pet bluemoon = new Pet {Name = "Blue Moon", Owner = terry};
Pet daisy = new Pet {Name = "Daisy", Owner = magnus};
// Create two lists.
List<Person> people = new List<Person> {magnus, terry, charlotte, arlene};
List<Pet> pets = new List<Pet> {barley, boots, whiskers, bluemoon, daisy};
var query = from person in people
where person.ID == 4
join pet in pets on person equals pet.Owner into personpets
from petOrNull in personpets.DefaultIfEmpty()
select new { Person=person, Pet = petOrNull};
foreach (var v in query )
{
Console.WriteLine("{0,-15}{1}", v.Person.FirstName + ":", (v.Pet == null ? "Does not Exist" : v.Pet.Name));
}
}
// This code produces the following output:
//
// Magnus: Daisy
// Terry: Barley
// Terry: Boots
// Terry: Blue Moon
// Charlotte: Whiskers
// Arlene:
now you are able to include elements from the left even if that element has no matches in the right, in our case we retrived Arlene even he has no matching in the right
here is the reference
How to: Perform Left Outer Joins (C# Programming Guide)
Upvotes: 13
Reputation: 121922
Take a look at this example. This query should work:
var leftFinal = from left in lefts
join right in rights on left equals right.Left into leftRights
from leftRight in leftRights.DefaultIfEmpty()
select new { LeftId = left.Id, RightId = left.Key==leftRight.Key ? leftRight.Id : 0 };
Upvotes: 53
Reputation: 568
Easy way is to use Let keyword. This works for me.
from AItem in Db.A
Let BItem = Db.B.Where(x => x.id == AItem.id ).FirstOrDefault()
Where SomeCondition
Select new YourViewModel
{
X1 = AItem.a,
X2 = AItem.b,
X3 = BItem.c
}
This is a simulation of Left Join. If each item in B table not match to A item , BItem return null
Upvotes: 3
Reputation: 4430
Here is a fairly easy to understand version using method syntax:
IEnumerable<JoinPair> outerLeft =
lefts.SelectMany(l =>
rights.Where(r => l.Key == r.Key)
.DefaultIfEmpty(new Item())
.Select(r => new JoinPair { LeftId = l.Id, RightId = r.Id }));
Upvotes: 8
Reputation: 216
I would like to add that if you get the MoreLinq extension there is now support for both homogenous and heterogeneous left joins now
http://morelinq.github.io/2.8/ref/api/html/Overload_MoreLinq_MoreEnumerable_LeftJoin.htm
example:
//Pretend a ClientCompany object and an Employee object both have a ClientCompanyID key on them
return DataContext.ClientCompany
.LeftJoin(DataContext.Employees, //Table being joined
company => company.ClientCompanyID, //First key
employee => employee.ClientCompanyID, //Second Key
company => new {company, employee = (Employee)null}, //Result selector when there isn't a match
(company, employee) => new { company, employee }); //Result selector when there is a match
EDIT:
In retrospect this may work, but it converts the IQueryable to an IEnumerable as morelinq does not convert the query to SQL.
You can instead use a GroupJoin as described here: https://stackoverflow.com/a/24273804/4251433
This will ensure that it stays as an IQueryable in case you need to do further logical operations on it later.
Upvotes: 6
Reputation: 19
Perform left outer joins in linq C# // Perform left outer joins
class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
class Child
{
public string Name { get; set; }
public Person Owner { get; set; }
}
public class JoinTest
{
public static void LeftOuterJoinExample()
{
Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };
Child barley = new Child { Name = "Barley", Owner = terry };
Child boots = new Child { Name = "Boots", Owner = terry };
Child whiskers = new Child { Name = "Whiskers", Owner = charlotte };
Child bluemoon = new Child { Name = "Blue Moon", Owner = terry };
Child daisy = new Child { Name = "Daisy", Owner = magnus };
// Create two lists.
List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
List<Child> childs = new List<Child> { barley, boots, whiskers, bluemoon, daisy };
var query = from person in people
join child in childs
on person equals child.Owner into gj
from subpet in gj.DefaultIfEmpty()
select new
{
person.FirstName,
ChildName = subpet!=null? subpet.Name:"No Child"
};
// PetName = subpet?.Name ?? String.Empty };
foreach (var v in query)
{
Console.WriteLine($"{v.FirstName + ":",-25}{v.ChildName}");
}
}
// This code produces the following output:
//
// Magnus: Daisy
// Terry: Barley
// Terry: Boots
// Terry: Blue Moon
// Charlotte: Whiskers
// Arlene: No Child
https://dotnetwithhamid.blogspot.in/
Upvotes: 1
Reputation: 61
Extension method that works like left join with Join syntax
public static class LinQExtensions
{
public static IEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
this IEnumerable<TOuter> outer, IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, TInner, TResult> resultSelector)
{
return outer.GroupJoin(
inner,
outerKeySelector,
innerKeySelector,
(outerElement, innerElements) => resultSelector(outerElement, innerElements.FirstOrDefault()));
}
}
just wrote it in .NET core and it seems to be working as expected.
Small test:
var Ids = new List<int> { 1, 2, 3, 4};
var items = new List<Tuple<int, string>>
{
new Tuple<int, string>(1,"a"),
new Tuple<int, string>(2,"b"),
new Tuple<int, string>(4,"d"),
new Tuple<int, string>(5,"e"),
};
var result = Ids.LeftJoin(
items,
id => id,
item => item.Item1,
(id, item) => item ?? new Tuple<int, string>(id, "not found"));
result.ToList()
Count = 4
[0]: {(1, a)}
[1]: {(2, b)}
[2]: {(3, not found)}
[3]: {(4, d)}
Upvotes: 6
Reputation: 31950
This is the general form (as already provided in other answers)
var c =
from a in alpha
join b in beta on b.field1 equals a.field1 into b_temp
from b_value in b_temp.DefaultIfEmpty()
select new { Alpha = a, Beta = b_value };
However here's an explanation that I hope will clarify what this actually means!
join b in beta on b.field1 equals a.field1 into b_temp
essentially creates a separate result set b_temp that effectively includes null 'rows' for entries on the right hand side (entries in 'b').
Then the next line:
from b_value in b_temp.DefaultIfEmpty()
..iterates over that result set, setting the default null value for the 'row' on the right hand side, and setting the result of the right hand side row join to the value of 'b_value' (i.e. the value that's on the right hand side,if there's a matching record, or 'null' if there isn't).
Now, if the right hand side is the result of a separate LINQ query, it will consist of anonymous types, which can only either be 'something' or 'null'. If it's an enumerable however (e.g. a List - where MyObjectB is a class with 2 fields), then it's possible to be specific about what default 'null' values are used for its properties:
var c =
from a in alpha
join b in beta on b.field1 equals a.field1 into b_temp
from b_value in b_temp.DefaultIfEmpty( new MyObjectB { Field1 = String.Empty, Field2 = (DateTime?) null })
select new { Alpha = a, Beta_field1 = b_value.Field1, Beta_field2 = b_value.Field2 };
This ensures that 'b' itself isn't null (but its properties can be null, using the default null values that you've specified), and this allows you to check properties of b_value without getting a null reference exception for b_value. Note that for a nullable DateTime, a type of (DateTime?) i.e. 'nullable DateTime' must be specified as the 'Type' of the null in the specification for the 'DefaultIfEmpty' (this will also apply to types that are not 'natively' nullable e.g double, float).
You can perform multiple left outer joins by simply chaining the above syntax.
Upvotes: 12
Reputation: 1
class Program
{
List<Employee> listOfEmp = new List<Employee>();
List<Department> listOfDepart = new List<Department>();
public Program()
{
listOfDepart = new List<Department>(){
new Department { Id = 1, DeptName = "DEV" },
new Department { Id = 2, DeptName = "QA" },
new Department { Id = 3, DeptName = "BUILD" },
new Department { Id = 4, DeptName = "SIT" }
};
listOfEmp = new List<Employee>(){
new Employee { Empid = 1, Name = "Manikandan",DepartmentId=1 },
new Employee { Empid = 2, Name = "Manoj" ,DepartmentId=1},
new Employee { Empid = 3, Name = "Yokesh" ,DepartmentId=0},
new Employee { Empid = 3, Name = "Purusotham",DepartmentId=0}
};
}
static void Main(string[] args)
{
Program ob = new Program();
ob.LeftJoin();
Console.ReadLine();
}
private void LeftJoin()
{
listOfEmp.GroupJoin(listOfDepart.DefaultIfEmpty(), x => x.DepartmentId, y => y.Id, (x, y) => new { EmpId = x.Empid, EmpName = x.Name, Dpt = y.FirstOrDefault() != null ? y.FirstOrDefault().DeptName : null }).ToList().ForEach
(z =>
{
Console.WriteLine("Empid:{0} EmpName:{1} Dept:{2}", z.EmpId, z.EmpName, z.Dpt);
});
}
}
class Employee
{
public int Empid { get; set; }
public string Name { get; set; }
public int DepartmentId { get; set; }
}
class Department
{
public int Id { get; set; }
public string DeptName { get; set; }
}
Upvotes: 0
Reputation: 340
(from a in db.Assignments
join b in db.Deliveryboys on a.AssignTo equals b.EmployeeId
//from d in eGroup.DefaultIfEmpty()
join c in db.Deliveryboys on a.DeliverTo equals c.EmployeeId into eGroup2
from e in eGroup2.DefaultIfEmpty()
where (a.Collected == false)
select new
{
OrderId = a.OrderId,
DeliveryBoyID = a.AssignTo,
AssignedBoyName = b.Name,
Assigndate = a.Assigndate,
Collected = a.Collected,
CollectedDate = a.CollectedDate,
CollectionBagNo = a.CollectionBagNo,
DeliverTo = e == null ? "Null" : e.Name,
DeliverDate = a.DeliverDate,
DeliverBagNo = a.DeliverBagNo,
Delivered = a.Delivered
});
Upvotes: -1
Reputation: 9
If you need to join and filter on something, that can be done outside of the join. Filter can be done after creating the collection.
In this case if I do this in the join condition I reduce the rows that are returned.
Ternary condition is used (= n == null ? "__" : n.MonDayNote,)
If the object is
null(so no match), then return what is after the?.__, in this case.Else, return what is after the
:,n.MonDayNote.
Thanks to the other contributors that is where I started with my own issue.
var schedLocations = (from f in db.RAMS_REVENUE_LOCATIONS
join n in db.RAMS_LOCATION_PLANNED_MANNING on f.revenueCenterID equals
n.revenueCenterID into lm
from n in lm.DefaultIfEmpty()
join r in db.RAMS_LOCATION_SCHED_NOTE on f.revenueCenterID equals r.revenueCenterID
into locnotes
from r in locnotes.DefaultIfEmpty()
where f.LocID == nLocID && f.In_Use == true && f.revenueCenterID > 1000
orderby f.Areano ascending, f.Locname ascending
select new
{
Facname = f.Locname,
f.Areano,
f.revenueCenterID,
f.Locabbrev,
// MonNote = n == null ? "__" : n.MonDayNote,
MonNote = n == null ? "__" : n.MonDayNote,
TueNote = n == null ? "__" : n.TueDayNote,
WedNote = n == null ? "__" : n.WedDayNote,
ThuNote = n == null ? "__" : n.ThuDayNote,
FriNote = n == null ? "__" : n.FriDayNote,
SatNote = n == null ? "__" : n.SatDayNote,
SunNote = n == null ? "__" : n.SunDayNote,
MonEmpNbr = n == null ? 0 : n.MonEmpNbr,
TueEmpNbr = n == null ? 0 : n.TueEmpNbr,
WedEmpNbr = n == null ? 0 : n.WedEmpNbr,
ThuEmpNbr = n == null ? 0 : n.ThuEmpNbr,
FriEmpNbr = n == null ? 0 : n.FriEmpNbr,
SatEmpNbr = n == null ? 0 : n.SatEmpNbr,
SunEmpNbr = n == null ? 0 : n.SunEmpNbr,
SchedMondayDate = n == null ? dMon : n.MondaySchedDate,
LocNotes = r == null ? "Notes: N/A" : r.LocationNote
}).ToList();
Func<int, string> LambdaManning = (x) => { return x == 0 ? "" : "Manning:" + x.ToString(); };
DataTable dt_ScheduleMaster = PsuedoSchedule.Tables["ScheduleMasterWithNotes"];
var schedLocations2 = schedLocations.Where(x => x.SchedMondayDate == dMon);
Upvotes: 0
Reputation: 402
There are three tables: persons, schools and persons_schools, which connects persons to the schools they study in. A reference to the person with id=6 is absent in the table persons_schools. However the person with id=6 is presented in the result lef-joined grid.
List<Person> persons = new List<Person>
{
new Person { id = 1, name = "Alex", phone = "4235234" },
new Person { id = 2, name = "Bob", phone = "0014352" },
new Person { id = 3, name = "Sam", phone = "1345" },
new Person { id = 4, name = "Den", phone = "3453452" },
new Person { id = 5, name = "Alen", phone = "0353012" },
new Person { id = 6, name = "Simon", phone = "0353012" }
};
List<School> schools = new List<School>
{
new School { id = 1, name = "Saint. John's school"},
new School { id = 2, name = "Public School 200"},
new School { id = 3, name = "Public School 203"}
};
List<PersonSchool> persons_schools = new List<PersonSchool>
{
new PersonSchool{id_person = 1, id_school = 1},
new PersonSchool{id_person = 2, id_school = 2},
new PersonSchool{id_person = 3, id_school = 3},
new PersonSchool{id_person = 4, id_school = 1},
new PersonSchool{id_person = 5, id_school = 2}
//a relation to the person with id=6 is absent
};
var query = from person in persons
join person_school in persons_schools on person.id equals person_school.id_person
into persons_schools_joined
from person_school_joined in persons_schools_joined.DefaultIfEmpty()
from school in schools.Where(var_school => person_school_joined == null ? false : var_school.id == person_school_joined.id_school).DefaultIfEmpty()
select new { Person = person.name, School = school == null ? String.Empty : school.name };
foreach (var elem in query)
{
System.Console.WriteLine("{0},{1}", elem.Person, elem.School);
}
Upvotes: 3
Reputation: 3178
Here's an example if you need to join more than 2 tables:
from d in context.dc_tpatient_bookingd
join bookingm in context.dc_tpatient_bookingm
on d.bookingid equals bookingm.bookingid into bookingmGroup
from m in bookingmGroup.DefaultIfEmpty()
join patient in dc_tpatient
on m.prid equals patient.prid into patientGroup
from p in patientGroup.DefaultIfEmpty()
Ref: https://stackoverflow.com/a/17142392/2343
Upvotes: 11
Reputation: 2694
This is a SQL syntax compare to LINQ syntax for inner and left outer joins. Left Outer Join:
http://www.ozkary.com/2011/07/linq-to-entity-inner-and-left-joins.html
"The following example does a group join between product and category. This is essentially the left join. The into expression returns data even if the category table is empty. To access the properties of the category table, we must now select from the enumerable result by adding the from cl in catList.DefaultIfEmpty() statement.
Upvotes: 2
Reputation: 503
An implementation of left outer join by extension methods could look like
public static IEnumerable<Result> LeftJoin<TOuter, TInner, TKey, Result>(
this IEnumerable<TOuter> outer, IEnumerable<TInner> inner
, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
, Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
{
if (outer == null)
throw new ArgumentException("outer");
if (inner == null)
throw new ArgumentException("inner");
if (outerKeySelector == null)
throw new ArgumentException("outerKeySelector");
if (innerKeySelector == null)
throw new ArgumentException("innerKeySelector");
if (resultSelector == null)
throw new ArgumentException("resultSelector");
return LeftJoinImpl(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer ?? EqualityComparer<TKey>.Default);
}
static IEnumerable<Result> LeftJoinImpl<TOuter, TInner, TKey, Result>(
IEnumerable<TOuter> outer, IEnumerable<TInner> inner
, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
, Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
{
var innerLookup = inner.ToLookup(innerKeySelector, comparer);
foreach (var outerElment in outer)
{
var outerKey = outerKeySelector(outerElment);
var innerElements = innerLookup[outerKey];
if (innerElements.Any())
foreach (var innerElement in innerElements)
yield return resultSelector(outerElment, innerElement);
else
yield return resultSelector(outerElment, default(TInner));
}
}
The resultselector then has to take care of the null elements. Fx.
static void Main(string[] args)
{
var inner = new[] { Tuple.Create(1, "1"), Tuple.Create(2, "2"), Tuple.Create(3, "3") };
var outer = new[] { Tuple.Create(1, "11"), Tuple.Create(2, "22") };
var res = outer.LeftJoin(inner, item => item.Item1, item => item.Item1, (it1, it2) =>
new { Key = it1.Item1, V1 = it1.Item2, V2 = it2 != null ? it2.Item2 : default(string) });
foreach (var item in res)
Console.WriteLine(string.Format("{0}, {1}, {2}", item.Key, item.V1, item.V2));
}
Upvotes: 20