How to Perform a Left Join, Group By, and Count with LINQ to SQL?

LINQ to SQL: Left Join, Group By, and Count

This example demonstrates how to perform a left join, group by, and count operation using LINQ to SQL, mirroring the functionality of a specific SQL query.

The SQL Query:

The following SQL query serves as our target for LINQ translation:

<code class="language-sql">SELECT p.ParentId, COUNT(c.ChildId)
FROM ParentTable p
LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
GROUP BY p.ParentId</code>

This query joins ParentTable and ChildTable, counts child records for each parent, and handles cases where a parent has no children (using a LEFT OUTER JOIN).

The LINQ to SQL Equivalent:

The equivalent LINQ to SQL query uses a join clause, DefaultIfEmpty() for the left join behavior, group by, and a Count() method with a predicate to count only non-null child IDs:

<code class="language-csharp">var query = from p in context.ParentTable
            join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1
            from j2 in j1.DefaultIfEmpty()
            group j2 by p.ParentId into grouped
            select new { 
                ParentId = grouped.Key, 
                ChildCount = grouped.Count(t => t.ChildId != null) 
            };</code>

This LINQ query achieves the same result as the SQL query. The into j1 and from j2 in j1.DefaultIfEmpty() combination correctly implements the left outer join. The group by clause groups the results by ParentId, and the Count() method, using a condition (t => t.ChildId != null), ensures that only existing child records are counted. The anonymous type new { ParentId, ChildCount } creates the final result set.

This clearly and concisely translates the SQL query's logic into a readable and maintainable LINQ expression.

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