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

LINQ to SQL: Implementing a Left Outer Join with Multiple Join Conditions

LINQ to SQL allows for the creation of complex queries using a C# syntax. One common task is implementing a left outer join with multiple join conditions.

To understand the need for including the join condition in the JOIN statement rather than the WHERE clause, consider the following SQL statement:

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

Here, even if the fact table (f) has no rows corresponding to a certain period, we still want that period included in the results. Placing the additional join condition in the WHERE clause would omit such periods from the results.

LINQ to SQL's solution involves using extension method syntax. The following code snippet demonstrates how:

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

Alternatively, you can employ a subquery:

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

By placing the join condition in the JOIN statement, we ensure that only periods with a corresponding fact satisfying the otherid = 17 condition are included in the results, while still performing a left outer join. This approach allows us to achieve the desired behavior without sacrificing the benefits of the left outer join.

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