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Practice

NULL and NOT NULL

In SQL, NULL represents missing or unknown data. It’s not the same as zero (0) or an empty string ('') — it simply means “no value.”


NULL Values​

A column might contain NULL when information is missing — for example, if a client hasn’t provided an email yet.


Check for NULL with IS NULL​

Use IS NULL or IS NOT NULL to check for missing values.

IS NULL returns TRUE when the value is NULL.

Find clients without an email
SELECT name
FROM clients
WHERE email IS NULL;

This query finds all clients who have provided an email address.


IS NOT NULL for Non-NULL Values​

The IS NOT NULL operator returns TRUE if the value is not NULL.

Find clients with an email
SELECT name
FROM clients
WHERE email IS NOT NULL;

The example above shows how to find clients who have provided an email.

You cannot use = to compare with NULL. Use IS NULL or IS NOT NULL.


NOT NULL Constraint​

The NOT NULL constraint prevents a column from having missing values.

For example, the query below creates a table with NOT NULL constraints on the id and name columns.

Create table with NOT NULL constraints
CREATE TABLE clients (
  id INT NOT NULL,
  name TEXT NOT NULL,
  email TEXT,
  signup_date TEXT
);

This ensures id and name are always provided.

Handling NULL properly ensures data integrity, accurate filtering and reporting, and fewer unexpected results in queries.

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