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.”

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

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

IS NULL returns TRUE when the value is NULL.

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SELECT name
FROM clients
WHERE email IS NULL;

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

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

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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.

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.

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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.