How to Perform a Left Outer Join with Multiple Join Conditions in LINQ to SQL?

LINQ to SQL: Perform left outer join using multiple join conditions

LINQ to SQL allows you to perform a left outer join using multiple join conditions to retrieve data from multiple tables, including rows from the left table even if there are no corresponding rows in the right table.

To implement a left outer join with multiple join conditions, you need to first establish the main join condition, which is usually the relationship between the primary keys of the table. Once you have your main join, you can add additional join conditions to further filter the results.

Sample SQL query you provided:

<code class="language-sql">SELECT f.value
FROM period as p
LEFT OUTER JOIN facts AS f ON p.id = f.periodid AND f.otherid = 17
WHERE p.companyid = 100</code>

Filter the rows in the 'period' table based on 'companyid' and retrieve the corresponding 'value' from the 'facts' table based on the 'id' and 'otherid' join conditions.

To convert this query to LINQ, you need to use the Join() method and the DefaultIfEmpty() method to handle outer joins. The correct LINQ implementation for a given SQL query is:

<code class="language-csharp">from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in fg.Where(f => f.otherid == 17).DefaultIfEmpty()
where p.companyid == 100
select f.value</code>

The

Where() clause is used to apply additional join conditions to otherid. The DefaultIfEmpty() method ensures that rows from the left table are included even if there are no matching rows in the right table.

Alternatively, you can use a subquery:

<code class="language-csharp">from p in context.Periods
join f in context.Facts on p.id equals f.periodid into fg
from fgi in (from f in fg
             where f.otherid == 17
             select f).DefaultIfEmpty()
where p.companyid == 100
select f.value</code>

Both methods produce the same results as the provided SQL query. By following these steps, you can efficiently perform a left outer join using multiple join conditions in LINQ to SQL.

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