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Remove the NOT NULL Constraint in Postgres

In relational databases such as PostgreSQL, constraints are crucial in maintaining the data integrity and enforcing specific rules on table columns. One such constraint is the “NOT NULL” constraint. In this tutorial, we will explore the concept of the NOT NULL constraint and provide a step-by-step guide on creating and removing it in PostgreSQL.

What Is a NOT NULL Constraint?

The NOT NULL constraint ensures that a column cannot contain null values which require every row in the table to have a non-null value for that particular column.

Tutorial Requirements:

To follow along with the instructions that are provided in this post, we assume that you have the following requirements:

  1. Installed PostgreSQL server on your machine
  2. Basic SQL knowledge
  3. Permission to create the database table objects such as constraints, functions, etc

Example Table:

Let us start by setting up a sample table with columns that contain a not null constraint. Take the following create table statement that sets up a basic table that resembles the wp_users table in WordPress:

CREATE TABLE wp_users (
    ID SERIAL PRIMARY KEY,
    user_login VARCHAR(60) NOT NULL,
    user_pass VARCHAR(255) NOT NULL,
    user_nicename VARCHAR(50) NOT NULL,
    user_email VARCHAR(100) NOT NULL,
    user_url VARCHAR(100),
    user_registered TIMESTAMP NOT NULL DEFAULT current_timestamp,
    user_activation_key VARCHAR(255),
    user_status INT NOT NULL DEFAULT 0,
    display_name VARCHAR(250) NOT NULL
);

From the given query, you will notice that most columns contain a NOT NULL constraint.

The most efficient method of creating a NOT NULL constraint in a given column is during the table creation. This ensures that all the data that are added to the table adheres to the rules of the constraint.

However, if you add a constraint to an existing table, the rules only apply to the data that is added after the constraint and not to any existing data. This can lead to inconsistencies, especially in large tables.

Add a NOT NULL Constraint

In some cases, you may want to add a NOT NULL constraint to an existing table. For example, you can use the ALTER TABLE command followed by the ALTER COLUMN clause.

The syntax is as follows:

ALTER TABLE table_name
ALTER COLUMN column_name SET NOT NULL;

Replace the table_name with your target table and the column_name with the target column to which you want to apply the constraint.

Verify the Constraint

You can verify that the constraint exists on a given table using the “\d” command in the PSQL utility.

\d table_name

Example:

sample_db=# \d wp_users;

The resulting output is as follows:

Remove the NOT NULL Constraint

You may also want to remove the NOT NULL constraint from a given column. For that, we can use the ALTER TABLE and ALTER COLUMN clauses as follows:

ALTER TABLE table_name
ALTER COLUMN column_name DROP NOT NULL;

For example, to remove the NOT NULL constraint from the user_url column of the wp_users table, we can run the following query:

ALTER TABLE wp_users ALTER COLUMN user_url DROP NOT NULL;

This should drop the constraint from the table.

Conclusion

This tutorial explored on how we can work with the NOT NOLL constraint in PostgreSQL. We also explored how to drop an existing NOT NULL constraint from a given column.

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Source: linuxhint.com

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