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

Implementing Left Outer Join with Multiple Join Conditions in LINQ to SQL

In situations where accessing data involves joining multiple tables with specified conditions, LINQ to SQL provides an effective means to perform such operations. This article delves into a specific scenario where a left outer join with multiple join conditions is required to retrieve data.

Initial Query and Challenge

Consider the following SQL query:

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

The task is to translate this query into a LINQ expression. While there are standard methods for performing left outer joins in LINQ (e.g., using DefaultIfEmpty()), incorporating the additional join condition f.otherid = 17 presents a challenge.

Solution: Placing the Join Condition Before DefaultIfEmpty()

The key to addressing this challenge is to introduce the join condition before invoking DefaultIfEmpty(). This can be achieved using either extension method syntax or a subquery:

// Using extension method syntax
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;

// Using 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;

Explanation of Join Condition Placement

The AND f.otherid = 17 condition is part of the join because it ensures that only rows from the facts table that meet this condition are joined with the corresponding rows from the period table. Placing this condition in the WHERE clause after the join would exclude any rows from the period table that do not have a matching row in the facts table, even if those rows would otherwise satisfy the condition. By specifying the condition in the join, we preserve the desired behavior.

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