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Apr 11, 2014 · Learn how to perform left outer join in C# LINQ to objects without using join-on-equals-into clauses. See examples, answers, and related questions from the Stack Overflow community.
Learn how to perform a left outer join in LINQ using method and query syntax. A left outer join returns all rows from the left table and the matched rows from the right table, with default values for the unmatched rows.
- In this article
- Methods
- Perform inner joins
- Perform grouped joins
- Perform left outer joins
- GeneratedCaptionsTabForHeroSec
of two data sources is the association of objects in one data source with objects that share a common attribute in another data source.
Joining is an important operation in queries that target data sources whose relationships to each other can't be followed directly. In object-oriented programming, joining could mean a correlation between objects that isn't modeled, such as the backwards direction of a one-way relationship. An example of a one-way relationship is a Student class that has a property of type Department that represents the major, but the Department class doesn't have a property that is a collection of Student objects. If you have a list of Department objects and you want to find all the students in each department, you could use a join operation to find them.
The join methods provided in the LINQ framework are
These methods perform equijoins, or joins that match two data sources based on equality of their keys. (For comparison, Transact-SQL supports join operators other than equals, for example the less than operator.) In relational database terms,
The following examples in this article use the common data sources for this area:
public required string FirstName { get; init; }
public required string LastName { get; init; }
public required int ID { get; init; }
public required GradeLevel Year { get; init; }
public required List Scores { get; init; }
In relational database terms, an
produces a result set in which each element of the first collection appears one time for every matching element in the second collection. If an element in the first collection has no matching elements, it doesn't appear in the result set. The
method, which is called by the join clause in C#, implements an inner join. The following examples show you how to perform four variations of an inner join:
A simple inner join that correlates elements from two data sources based on a simple key.
An inner join that correlates elements from two data sources based on a
key. A composite key, which is a key that consists of more than one value, enables you to correlate elements based on more than one property.
The group join is useful for producing hierarchical data structures. It pairs each element from the first collection with a set of correlated elements from the second collection.
Each element of the first collection appears in the result set of a group join regardless of whether correlated elements are found in the second collection. In the case where no correlated elements are found, the sequence of correlated elements for that element is empty. The result selector therefore has access to every element of the first collection. This differs from the result selector in a non-group join, which cannot access elements from the first collection that have no match in the second collection.
A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection. You can use LINQ to perform a left outer join by calling the
method on the results of a group join.
The following example demonstrates how to use the
method on the results of a group join to perform a left outer join.
The first step in producing a left outer join of two collections is to perform an inner join by using a group join. (See
for an explanation of this process.) In this example, the list of Department objects is inner-joined to the list of Student objects based on a Department object's ID that matches the student's DepartmentID.
Learn how to perform inner joins, grouped joins, and left outer joins in LINQ using the Join, GroupJoin, and Enumerable.Join methods. See examples of joining data sources based on key selector functions and composite keys.
Learn how to perform join operations in LINQ to combine data from two or more sources based on a common key or condition. See examples of inner join, group join, and left outer join using the Join, GroupJoin, and SelectMany methods.
Mar 6, 2024 · In this article, we'll discuss how to create an outer join in LINQ, including the left and right outer joins.
Jun 7, 2024 · In this post, we’ll help in your LINQ-mastering quest by covering the LINQ join operator. We’ll start the post with a definition of LINQ itself, so we’re all on the same page. After that, you’ll see an explanation of join operations in LINQ.
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How to perform a LEFT OUTER JOIN in LINQ?
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What are join methods in LINQ?
In query expression syntax, a join (C#) or Join (Visual Basic) clause translates to an invocation of Join. In relational database terms, the Join method implements an inner equijoin. 'Inner' means that only elements that have a match in the other sequence are included in the results.
