What's the Difference Between LINQ's Join and GroupJoin Methods?

Comparison of Join and GroupJoin in LINQ to Entities

Understanding GroupJoin

GroupJoin, like Join, is a LINQ method for combining multiple data sources. However, GroupJoin produces results with a different structure than Join.

Join:

• Perform an inner join and only generate rows where there are matches in both joined tables.
• Result: A merged table containing columns from both tables.

GroupJoin:

• Perform an outer join to match rows in the first table with rows in the second table.
• Result: A collection of groups, where each group contains a row from the first table and its matching row from the second table (or an empty list if no match exists).

Grammar

Query syntax:

Join:

<code>from p in Parent
join c in Child on p.Id equals c.Id
select new { p.Value, c.ChildValue }</code>

GroupJoin:

<code>from p in Parent
join c in Child on p.Id equals c.Id into g
select new { Parent = p, Children = g }</code>

Method syntax (less commonly used):

Join:

<code>Parent.Join(Child, p => p.Id, c => c.Id, (p, c) => new { p.Value, c.ChildValue })</code>

GroupJoin:

<code>Parent.GroupJoin(Child, p => p.Id, c => c.Id, (p, c) => new { Parent = p, Children = c })</code>

Use cases

1. Generate flat outer connection:

• GroupJoin can be used to create a flattened outer join that contains every row in the first table, even if there are no matching rows in the second table. This is achieved by flattening the group:

<code>from p in Parent
join c in Child on p.Id equals c.Id into g
from c in g.DefaultIfEmpty()
select new { Parent = p.Value, Child = c?.ChildValue }</code>

2. Keep order:

• Join can be used to filter a list while maintaining the order of elements. By using an external sequence as the first list, you can maintain the order of elements from the second list:

<code>from id in ids
join p in parents on id equals p.Id
select p</code>

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