How to Perform a Left Join, Group By, and Count Non-Null Child Records using LINQ?

Use LINQ to implement left join, grouping and non-empty sub-record counting

In SQL, the following query performs a left join between two tables, ParentTable and ChildTable, and counts the number of child records associated with each parent record:

<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>

Convert this query to LINQ to SQL using the following steps:

1. Perform a left join using the Join and DefaultIfEmpty methods:

<code class="language-csharp">from p in context.ParentTable
join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1
from j2 in j1.DefaultIfEmpty()</code>

2. Group results by parent ID:

<code class="language-csharp">group j2 by p.ParentId into grouped</code>

3. Count the number of child records associated with each parent record:

<code class="language-csharp">select new { ParentId = grouped.Key, Count = grouped.Count() }</code>

However, the above code will return the total number of child records for each parent record, including null values. To count only non-empty sub-records, modify the Count expression as follows:

<code class="language-csharp">select new { ParentId = grouped.Key, Count = grouped.Where(t => t.ChildId != null).Count() }</code>

This will ensure that only child records that ChildId are not empty are counted. The final LINQ query will accurately reflect the logic of the SQL query and effectively count non-empty sub-records.

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