Common mistake

Why = NULL Never Matches in SQL

A comparison to NULL returns 'unknown', not true, so WHERE quietly drops the row.

The symptom

You filter for missing values with where phone = null (or where status != 'shipped') and get back fewer rows than expected or none at all. No error, just a wrong answer.

Why it happens

In SQL, NULL is a marker for a missing or unknown value. Ordinary comparisons involving NULL evaluate to the third SQL truth value, UNKNOWN, so phone = null is not true. A WHERE clause keeps only rows whose condition is true, so those rows fall out.

The same trap bites <> / !=: a row whose value is NULL is neither equal nor unequal to your target, so a plain status <> 'shipped' skips every null-status row. Write status is distinct from 'shipped' instead, which treats NULL as just another value and keeps those rows.

The fix

Use IS NULL and IS NOT NULL when you need to test whether a value is missing. Some Garden Shop customers have no phone number on file; run the pair below and see which version finds them.

Wrong: = null is never true

select customer_id, first_name, phone
from customers
where phone = null

Right: is null

select customer_id, first_name, phone
from customers
where phone is null

Rule of thumb

When the question is whether a value is missing, use IS NULL / IS NOT NULL. When you need a comparison that treats nulls as comparable values, consider IS DISTINCT FROM.