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Standard Join gives you a flat list. GroupJoin gives you a hierarchical list, where each outer item is paired with a collection of its matching inner items.
Think of a list of Departments and its Employees. A GroupJoin will give you a sequence where each Department object contains a list of its Employees. This is exactly how we usually think about data in the real world.
var query = departments.GroupJoin(
employees,
dept => dept.Id,
emp => emp.DeptId,
(dept, empGroup) => new {
DepartmentName = dept.Name,
Staff = empGroup // staff is an IEnumerable<Employee>
}
);
In SQL, there is no direct equivalent to GroupJoin because SQL only returns flat sets. EF Core translates this into a LEFT OUTER JOIN and then reshapes the data into objects on the client side.
Q: "When should I use GroupJoin instead of GroupBy?"
Architect Answer: "Use **GroupJoin** if you already have two separate lists. Use **GroupBy** if you have ONE list that you want to split into categories. GroupJoin is essentially a 'Join then GroupBy' in one high-performance operation."
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On this page
1. Hierarchy Pattern 2. Relationship with SQL 3. Architect Insight