add a column postgres code example

Example 1: postgresql add column with constraint

ALTER TABLE customers 
ADD COLUMN contact_name VARCHAR NOT NULL;

Example 2: postgresql insert column

ALTER TABLE table_name
ADD COLUMN new_column_name data_type constraint;

Example 3: postgres how to add field created at

CREATE TABLE my_table
(
	id serial NOT NULL,
    user_name text,
	created_at timestamp with time zone DEFAULT CURRENT_TIMESTAMP //here
)