PostgreSQL Tutorial: NOT NULL Constraint

Summary: in this tutorial, you will learn about PostgreSQL not-null constraint to ensure the values of a column are not null.

Introduction to NULL

In database theory, NULL represents unknown or information missing. NULL is not the same as an empty string or the number zero.

Suppose that you need to insert an email address of a contact into a table. You can request his or her email address. However, if you don’t know whether the contact has an email address or not, you can insert NULL into the email address column. In this case, NULL indicates that the email address is not known at the time of recording.

NULL is very special. It does not equal anything, even itself. The expression NULL = NULL returns NULL because it makes sense that two unknown values should not be equal.

To check if a value is NULL or not, you use the IS NULL boolean operator. For example, the following expression returns true if the value in the email address is NULL.

email_address IS NULL

The IS NOT NULL operator negates the result of the IS NULL operator.

PostgreSQL NOT NULL constraint

To control whether a column can accept NULL, you use the NOT NULL constraint:

CREATE TABLE table_name(
   ...
   column_name data_type NOT NULL,
   ...
);

If a column has a NOT NULL constraint, any attempt to insert or update NULL in the column will result in an error.

Declaring NOT NULL columns

The following CREATE TABLE statement creates a new table name invoices with the not-null constraints.

CREATE TABLE invoices(
  id SERIAL PRIMARY KEY,
  product_id INT NOT NULL,
  qty numeric NOT NULL CHECK(qty > 0),
  net_price numeric CHECK(net_price > 0) 
);

This example uses the NOT NULL keywords that follow the data type of the product_id and qty columns to declare NOT NULL constraints.

Note that a column can have multiple constraints such as NOT NULL, check, unique, foreign key appeared next to each other. The order of the constraints is not important. PostgreSQL can check the constraint in the list in any order.

If you use NULL instead of NOT NULL, the column will accept both NULL and non-NULL values. If you don’t explicitly specify NULL or NOT NULL, it will accept NULL by default.

Adding NOT NULL Constraint to existing columns

To add the NOT NULL constraint to a column of an existing table, you use the following form of the ALTER TABLE statement:

ALTER TABLE table_name
  ALTER COLUMN column_name SET NOT NULL;

To add set multiple NOT NULL constraint to multiple columns, you use the following syntax:

ALTER TABLE table_name
  ALTER COLUMN column_name_1 SET NOT NULL,
  ALTER COLUMN column_name_2 SET NOT NULL,
  ...;

Let’s take a look at the following example.

First, create a new table called production orders ( production_orders):

CREATE TABLE production_orders (
	id SERIAL PRIMARY KEY,
	description VARCHAR (40) NOT NULL,
	material_id VARCHAR (16),
	qty NUMERIC,
	start_date DATE,
	finish_date DATE
);

Next, insert a new row into the production_orders table:

INSERT INTO production_orders (description)
VALUES('Make for Infosys inc.');

Then, to make sure that the qty field is not null, you can add the not-null constraint to the qty column. However, the column already contains data. If you try to add the not-null constraint, PostgreSQL will issue an error.

To add the NOT NULL constraint to a column that already contains NULL, you need to update NULL to non-NULL first, like this:

UPDATE production_orders
SET qty = 1;

The values in the qty column are updated to one. Now, you can add the NOT NULL constraint to the qty column:

ALTER TABLE production_orders 
  ALTER COLUMN qty
    SET NOT NULL;

After that, you can update the not-null constraints for material_id, start_date, and finish_date columns:

UPDATE production_orders
SET material_id = 'ABC',
    start_date = '2015-09-01',
    finish_date = '2015-09-01';

Add not-null constraints to multiple columns:

ALTER TABLE production_orders 
  ALTER COLUMN material_id SET NOT NULL,
  ALTER COLUMN start_date SET NOT NULL,
  ALTER COLUMN finish_date SET NOT NULL;

Finally, attempt to update values in the qty column to NULL:

UPDATE production_orders
SET qty = NULL;

PostgreSQL issued an error message:

ERROR:  null value in column "qty" violates not-null constraint
DETAIL:  Failing row contains (1, make for infosys inc., ABC, null, 2015-09-01, 2015-09-01).

The special case of NOT NULL constraint

Besides the NOT NULL constraint, you can use a CHECK constraint to force a column to accept not NULL values. The NOT NULL constraint is equivalent to the following CHECK constraint:

CHECK(column IS NOT NULL)

This is useful because sometimes you may want either column a or b is not null, but not both.

For example, you may want either username or email column of the user tables is not null or empty. In this case, you can use the CHECK constraint as follows:

CREATE TABLE users (
 id serial PRIMARY KEY,
 username VARCHAR (50),
 password VARCHAR (50),
 email VARCHAR (50),
 CONSTRAINT username_email_notnull CHECK (
   NOT (
     ( username IS NULL  OR  username = '' )
     AND
     ( email IS NULL  OR  email = '' )
   )
 )
);

The following statement works.

INSERT INTO users (username, email)
VALUES
	('user1', NULL),
	(NULL, 'email1@example.com'),
	('user2', 'email2@example.com'),
	('user3', '');

However, the following statement will not work because it violates the CHECK constraint:

INSERT INTO users (username, email)
VALUES
	(NULL, NULL),
	(NULL, ''),
	('', NULL),
	('', '');

ERROR:  new row for relation "users" violates check constraint "username_email_notnull"

Summary

• Use the NOT NULL constraint for a column to enforce a column not accept NULL. By default, a column can hold NULL.
• To check if a value is NULL or not, you use the IS NULL operator. The IS NOT NULL negates the result of the IS NULL.
• Never use equal operator = to compare a value with NULL because it always returns NULL.