Oracle NOT EXISTS

Summary: in this tutorial, you learn how to use the Oracle NOT EXISTS operator to subtract one set of data from another.

Introduction to the Oracle NOT EXISTS operator

The NOT EXISTS operator works the opposite of the EXISTS operator. We often use the NOT EXISTS operator with a subquery to subtract one set of data from another.

Consider the following statement that uses the NOT EXISTS operator:

SELECT
    *
FROM
    table_name
WHERE
    NOT EXISTS (subquery);

The NOT EXISTS operator returns true if the subquery returns no row. Otherwise, it returns false.

Note that the NOT EXISTS operator returns false if the subquery returns any rows with a NULL value.

Oracle NOT EXISTS examples

See the following customers and orders tables in the sample database:

The following statement finds all customers who have no order:

SELECT
    name
FROM
    customers
WHERE
    NOT EXISTS (
        SELECT
            NULL
        FROM
            orders
        WHERE
            orders.customer_id = customers.customer_id
    )
ORDER BY
    name;

To archive the customers who have no order, you use the following statement:

CREATE TABLE customers_archive AS
SELECT * 
FROM
    customers
WHERE
    NOT EXISTS (
        SELECT
            NULL
        FROM
            orders
        WHERE
            orders.customer_id = customers.customer_id
    );

To update the credit limit of customers who have no order in 2017, you use the following UPDATE statement:

UPDATE
    customers
SET
    credit_limit = 0
WHERE
    NOT EXISTS(
        SELECT
            NULL
        FROM
            orders
        WHERE
            orders.customer_id = customers.customer_id
            AND EXTRACT(
                YEAR
            FROM
                order_date
            
    );

And to delete all customers who had no order in 2016 and 2017 from the customers table, you use the following DELETE statement:

DELETE
FROM
    customers
WHERE
    NOT EXISTS(
        SELECT
            NULL
        FROM
            orders
        WHERE
            orders.customer_id = customers.customer_id
            AND EXTRACT(
                YEAR FROMorder_date
            ) IN(
                2016,
                2017
            )
    );

Oracle NOT EXISTS vs. NOT IN

The following statement uses the IN operator with a subquery:

SELECT
	*
FROM
	table_name
WHERE
	id IN(subquery);

Suppose the subquery returns four values 1, 2, 3, and NULL. You can rewrite the whole query above as follows:

SELECT
    *
FROM
    table_name
WHERE
    id = 1
    OR id = 2  
    OR id = 3
    OR id = NULL;

The following expression always returns a NULL value because a NULL value cannot compare to anything.

id = NULL

Therefore, the following expression returns a NULL value if any row in the result set of the subquery is NULL.

id NOT IN (subquery)

In contrast, NULL does not affect the result of the NOT EXIST operator because the NOT EXISTS operator solely checks the existence of rows in the subquery:

SELECT
    *
FROM
    table_name
WHERE
    NOT EXISTS(subquery);

In conclusion, the NOT EXISTS and NOT IN behave differently when there are null values involved.

In this tutorial, you have learned how to use the Oracle NOT EXISTS operator to subtract one set of data from another.