What is an anti join in SQL?

What is an anti join in SQL?

In SQL, an anti-join is a type of join that returns only the rows from one table that do not have any matching rows in the other table. This is useful for identifying rows that are present in one table but not in the other.

There are two main ways to perform an anti-join in SQL:

1. Using the LEFT JOIN and IS NULL operator
2. Using the NOT IN operator.

Here is an example of how to use each of these methods:

-- Using the LEFT JOIN and IS NULL operator
SELECT u.id
FROM users u
LEFT JOIN orders o
ON u.id = o.user_id
WHERE o.user_id IS NULL;

-- Using the NOT IN operator
SELECT id
FROM users
WHERE id NOT IN (SELECT user_id FROM orders);

In both of these examples, the query returns the id values from the users table that do not have any matching user_id values in the orders table. This allows you to identify users who have not placed any orders.

The results of an anti-join can be affected by the NULL values in the tables being joined.

In the first example, the LEFT JOIN and IS NULL operator is used to handle NULL values, but in the second example, the NOT IN operator does not handle NULL values.

This means that the second example may not return all the rows that do not have matching rows in the other table if the NULL values are not handled properly.

Semi-join vs. anti-join

A semi-join is a type of join that returns only the rows from one table that have matching rows in the other table. This is useful for filtering the rows in one table based on the values in the other table.

An anti-join, on the other hand, is a type of join that returns only the rows from one table that do not have any matching rows in the other table. This is useful for identifying rows that are present in one table but not in the other.

Here is an example of how to perform a semi-join and an anti-join in SQL:

-- Semi-join
SELECT u.id
FROM users u
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);

-- Anti-join
SELECT u.id
FROM users u
WHERE NOT EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);

Conclusion

In SQL, an anti-join is a type of join that returns only the rows from one table that do not have any matching rows in the other table. This is useful for identifying rows that are present in one table but not in the other.