IS NULL

In the database world, NULL means missing information or not applicable. NULL is not a value, therefore, you cannot compare it with other values like numbers or strings.

The comparison of NULL with a value will always result in NULL. Additionally, NULL is not equal to NULL so the following expression returns NULL.

SELECT null = null AS result;

Output:

result
--------
 null
(1 row)

IS NULL operator

To check if a value is NULL or not, you cannot use the equal to (=) or not equal to (<>) operators. Instead, you use the IS NULL operator.

Here’s the basic syntax of the IS NULL operator.

value IS NULL

The IS NULL operator returns true if the value is NULL or false otherwise. To negate the IS NULL operator, you use the IS NOT NULL operator.

value IS NOT NULL

The IS NOT NULL operator returns true if the value is not NULL or false otherwise.

1) Basic IS NULL operator example

The following example uses the IS NULL operator to find the addresses from the address table that the address2 column contains NULL

SELECT address, address2
FROM address
WHERE address2 IS NULL;

2) Using the IS NOT NULL operator example

The following example uses the IS NOT NULL operator to retrieve the address that has the address2 not NULL.

SELECT address, address2
FROM address
WHERE address2 IS NOT NULL;

Notice that the address2 is empty, not NULL. This is a good example of bad practice when it comes to storing empty strings and NULL in the same column.