NOT IN vs NOT EXISTS

If the execution planner says they're the same, they're the same. Use whichever one will make your intention more obvious -- in this case, the second.

Actually, I believe this would be the fastest:

SELECT ProductID, ProductName 
    FROM Northwind..Products p  
          outer join Northwind..[Order Details] od on p.ProductId = od.ProductId)
WHERE od.ProductId is null

In your specific example they are the same, because the optimizer has figured out what you are trying to do is the same in both examples. But it is possible that in non-trivial examples the optimizer may not do this, and in that case there are reasons to prefer one to other on occasion.

NOT IN should be preferred if you are testing multiple rows in your outer select. The subquery inside the NOT IN statement can be evaluated at the beginning of the execution, and the temporary table can be checked against each value in the outer select, rather than re-running the subselect every time as would be required with the NOT EXISTS statement.

If the subquery must be correlated with the outer select, then NOT EXISTS may be preferable, since the optimizer may discover a simplification that prevents the creation of any temporary tables to perform the same function.

Database table model

Let’s assume we have the following two tables in our database, that form a one-to-many table relationship.

The student table is the parent, and the student_grade is the child table since it has a student_id Foreign Key column referencing the id Primary Key column in the student table.

The student table contains the following two records:

And, the student_grade table stores the grades the students received:

SQL EXISTS

Let’s say we want to get all students that have received a 10 grade in Math class.

If we are only interested in the student identifier, then we can run a query like this one:

SELECT
    student_grade.student_id
FROM
    student_grade
WHERE
    student_grade.grade = 10 AND
    student_grade.class_name = 'Math'
ORDER BY
    student_grade.student_id

But, the application is interested in displaying the full name of a student, not just the identifier, so we need info from the student table as well.

In order to filter the student records that have a 10 grade in Math, we can use the EXISTS SQL operator, like this:

SELECT
    id, first_name, last_name
FROM
    student
WHERE EXISTS (
    SELECT 1
    FROM
        student_grade
    WHERE
        student_grade.student_id = student.id AND
        student_grade.grade = 10 AND
        student_grade.class_name = 'Math'
)
ORDER BY id

When running the query above, we can see that only the Alice row is selected:

The outer query selects the student row columns we are interested in returning to the client. However, the WHERE clause is using the EXISTS operator with an associated inner subquery.

The EXISTS operator returns true if the subquery returns at least one record and false if no row is selected. The database engine does not have to run the subquery entirely. If a single record is matched, the EXISTS operator returns true, and the associated other query row is selected.

The inner subquery is correlated because the student_id column of the student_grade table is matched against the id column of the outer student table.

SQL NOT EXISTS

Let’s consider we want to select all students that have no grade lower than 9. For this, we can use NOT EXISTS, which negates the logic of the EXISTS operator.

Therefore, the NOT EXISTS operator returns true if the underlying subquery returns no record. However, if a single record is matched by the inner subquery, the NOT EXISTS operator will return false, and the subquery execution can be stopped.

To match all student records that have no associated student_grade with a value lower than 9, we can run the following SQL query:

SELECT
    id, first_name, last_name
FROM
    student
WHERE NOT EXISTS (
    SELECT 1
    FROM
        student_grade
    WHERE
        student_grade.student_id = student.id AND
        student_grade.grade < 9
)
ORDER BY id

When running the query above, we can see that only the Alice record is matched:

So, the advantage of using the SQL EXISTS and NOT EXISTS operators is that the inner subquery execution can be stopped as long as a matching record is found.

If the optimizer says they are the same then consider the human factor. I prefer to see NOT EXISTS :)

It depends..

SELECT x.col
FROM big_table x
WHERE x.key IN( SELECT key FROM really_big_table );

would not be relatively slow the isn't much to limit size of what the query check to see if they key is in. EXISTS would be preferable in this case.

But, depending on the DBMS's optimizer, this could be no different.

As an example of when EXISTS is better

SELECT x.col
FROM big_table x
WHERE EXISTS( SELECT key FROM really_big_table WHERE key = x.key);
  AND id = very_limiting_criteria