PostgreSQL Tutorial: ADD COLUMN: Add One Or More Columns To a Table

August 2, 2023

Summary: In this tutorial, you will learn how to use the PostgreSQL ADD COLUMN statement to add one or more columns to an existing table.

Table of Contents

PostgreSQL Add Column

Introduction to the PostgreSQL ADD COLUMN statement

To add a new column to an existing table, you use the ALTER TABLE ADD COLUMN statement as follows:

ALTER TABLE table_name
ADD COLUMN new_column_name data_type constraint;

In this syntax:

  • First, specify the name of the table that you want to add a new column to after the ALTER TABLE keyword.
  • Second, specify the name of the new column as well as its data type and constraint after the ADD COLUMN keywords.

When you add a new column to the table, PostgreSQL appends it at the end of the table. PostgreSQL has no option to specify the position of the new column in the table.

To add multiple columns to an existing table, you use multiple ADD COLUMN clauses in the ALTER TABLE statement as follows:

ALTER TABLE table_name
ADD COLUMN column_name1 data_type constraint,
ADD COLUMN column_name2 data_type constraint,
...
ADD COLUMN column_namen data_type constraint;

PostgreSQL ADD COLUMN statement examples

The following CREATE TABLE statement creates a new table named customers with two columns: id and customer_name:

DROP TABLE IF EXISTS customers CASCADE;

CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    customer_name VARCHAR NOT NULL
);

The following statement uses the ALTER TABLE ADD COLUMN statement to add the phone column to the customers table:

ALTER TABLE customers 
ADD COLUMN phone VARCHAR;

And the following statement adds the fax and email columns to the customers table:

ALTER TABLE customers
ADD COLUMN fax VARCHAR,
ADD COLUMN email VARCHAR;

To view the structure of the customers table in the psql tool, you can use the \d command like this:

\d customers

img

As can be seen clearly from the output, the phone, fax, and email columns appeared at the end of the column list of the customers table.

Add a column with the NOT NULL constraint to a table that already has data

The following statement inserts data into the customers table.

INSERT INTO 
   customers (customer_name)
VALUES
   ('Apple'),
   ('Samsung'),
   ('Sony');

Suppose that you want to add the contact_name column to the customers table:

ALTER TABLE customers 
ADD COLUMN contact_name VARCHAR NOT NULL;

PostgreSQL issued an error:

ERROR:  column "contact_name" contains null values

This is because the contact_name column has the NOT NULL constraint. When PostgreSQL added the column, this new column receive NULL, which violates the NOT NULL constraint.

To solve this problem…

First, add the column without the NOT NULL constraint:

ALTER TABLE customers 
ADD COLUMN contact_name VARCHAR;

Second, update the values in the contact_name column.

UPDATE customers
SET contact_name = 'John Doe'
WHERE id = 1;

UPDATE customers
SET contact_name = 'Mary Doe'
WHERE id = 2;

UPDATE customers
SET contact_name = 'Lily Bush'
WHERE id = 3;

Third, set the NOT NULL constraint for the contact_name column.

ALTER TABLE customers
ALTER COLUMN contact_name SET NOT NULL;

In this tutorial, you have learned how to use the PostgresSQL ADD COLUMN statement to add one or more columns to a table.

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