How to Efficiently Implement IN Subqueries in LINQ to SQL?

Handling IN Sub-Queries in LINQ to SQL

Utilizing IN sub-queries in LINQ to SQL can be an efficient way to correlate data from multiple tables. Here's how to approach it:

General IN Query Implementation:

To implement an IN query in LINQ to SQL, follow this syntax:

var q = from t1 in table1
        let t2s = from t2 in table2
                  where <Conditions for table2>
                  select t2.KeyField
        where t2s.Contains(t1.KeyField)
        select t1;

Specific Example:

Let's revisit the example SQL query:

SELECT f.* 
FROM Foo f
WHERE f.FooId IN (
    SELECT fb.FooId
    FROM FooBar fb
    WHERE fb.BarId = 1000
)

In LINQ to SQL, this would translate to:

var query = from f in db.Foo
            let fubars = from fb in db.FooBar
                          where fb.BarId == 1000
                          select fb.FooId
            where fubars.Contains(f.FooId)
            select f;

Additional Considerations:

For EXISTS queries, you can use the Any() method instead of Contains():

var q = from t1 in table1
        let t2s = from t2 in table2
                  where <Conditions for table2>
                  select t2.KeyField
        where t2s.Any(t1.KeyField)
        select t1;

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