PostgreSQL NOT NULL Constraints

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

Introduction to NULL

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

Suppose you need to insert the 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 unknown at the recording time.

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 NULLCode language: PHP (php)

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

PostgreSQL NOT NULL constraints

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

CREATE TABLE table_name(
   ...
   column_name data_type NOT NULL,
   ...
);Code language: PHP (php)

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) 
);Code language: SQL (Structured Query Language) (sql)

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 appearing next to each other. The order of the constraints is not important. PostgreSQL may check constraints 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 Constraints 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;Code language: SQL (Structured Query Language) (sql)

To add multiple NOT NULL constraints 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,
...;Code language: SQL (Structured Query Language) (sql)

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
);Code language: SQL (Structured Query Language) (sql)

Next, insert a new row into the production_orders table:

INSERT INTO production_orders (description)
VALUES('Make for Infosys inc.');Code language: SQL (Structured Query Language) (sql)

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;Code language: SQL (Structured Query Language) (sql)

The values in the qty column is 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;Code language: SQL (Structured Query Language) (sql)

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';Code language: SQL (Structured Query Language) (sql)

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;Code language: SQL (Structured Query Language) (sql)

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

UPDATE production_orders
SET qty = NULL;Code language: SQL (Structured Query Language) (sql)

PostgreSQL issued an error message:

[Err] 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).Code language: JavaScript (javascript)

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)Code language: SQL (Structured Query Language) (sql)

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 = ''
      )
    )
  )
);Code language: SQL (Structured Query Language) (sql)

The following statement works.

INSERT INTO users (username, email)
VALUES
	('user1', NULL),
	(NULL, '[email protected]'),
	('user2', '[email protected]'),
	('user3', '');Code language: SQL (Structured Query Language) (sql)

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

INSERT INTO users (username, email)
VALUES
	(NULL, NULL),
	(NULL, ''),
	('', NULL),
	('', '');Code language: SQL (Structured Query Language) (sql)

[Err] ERROR:  new row for relation "users" violates check constraint "username_email_notnull"Code language: JavaScript (javascript)

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.

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