Lesson 5: Joining Collections with LINQ

LINQ joins let you combine related data from multiple collections, similar to SQL joins. This is useful when working with object graphs or in-memory relational data.

Inner Join with Join

Join pairs elements from two collections when a key matches.

var students = new[]
{
    new { Id = 1, Name = "Ava" },
    new { Id = 2, Name = "Leo" }
};

var grades = new[]
{
    new { StudentId = 1, Grade = 90 },
    new { StudentId = 2, Grade = 82 }
};

var report = students.Join(
    grades,
    student => student.Id,
    grade => grade.StudentId,
    (student, grade) => new { student.Name, grade.Grade }
);

Left Join with GroupJoin

GroupJoin can simulate a left join by pairing each left item with zero or more right items.

var courses = new[]
{
    new { Id = 1, Title = "C# Basics" },
    new { Id = 2, Title = "LINQ Deep Dive" }
};

var enrollments = new[]
{
    new { CourseId = 2, Student = "Ava" }
};

var courseRoster = courses.GroupJoin(
    enrollments,
    course => course.Id,
    enrollment => enrollment.CourseId,
    (course, studentGroup) => new
    {
        course.Title,
        Students = studentGroup.Select(s => s.Student)
    }
);

Cross Join with SelectMany

SelectMany can produce a Cartesian product when each item is paired with every item in another sequence.

var colors = new[] { "Red", "Blue" };
var sizes = new[] { "S", "M", "L" };

var variants = colors.SelectMany(color => sizes, (color, size) => new { color, size });

Real-World Match

Joining helps when you need combined results like student names with grades, order details with products, or customers with invoices.

🧠 Quick Check — Lesson 5

Which LINQ method is best for an inner join?

Lesson Summary

✅

Join performs an inner join between two sequences using matching keys.

✅

GroupJoin is useful for left-join style grouping of related items.

✅

SelectMany can flatten results and generate a cross join.

✅

LINQ joins are excellent when working with normalized in-memory data structures.

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