How to Translate SQL's IN Subqueries into LINQ to SQL?

Handling IN Sub-Queries with LINQ to SQL

In LINQ to SQL, you may encounter the need to handle IN sub-queries, a common operation in SQL. Let's consider a scenario:

Question:

How can we translate the following SQL query into LINQ to SQL?

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

Answer:

To implement such a query in LINQ to SQL, we need to use two concepts:

1. Sub-query as a Collection of Keys:

We start by defining the sub-query as a collection of keys that satisfy the condition in the IN clause.

var fooBarIds = from fb in context.FooBar
                where fb.BarId == 1000
                select fb.FooId;

2. Query with Contains Method:

Next, we can use the Contains method on the collection of keys to check if a given FooId is present.

var result = from f in context.Foo
             where fooBarIds.Contains(f.FooId)
             select f;

This query will return all rows from the Foo table where the FooId exists in the collection of FooIds retrieved from the sub-query.

Extended Use Cases:

The same principles can be applied to implement other sub-query operations, such as EXISTS:

// EXISTS
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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