Add a column with a default value to an existing table in postgresql

Question

Question:
Is there a postgres query to add a new column to an existing table and to automatically populate that column for all rows of the table with a certain value, let's say "A1", just once, as the column is created, so that I can still set the DEFAULT value of the column to another value, let's say "B2"?

Just to be clear, I am looking for something like this:

Given my_table:

name   |   work
------------------------
bob    |  fireman
carl   |  teacher
alice  |  policeman

my query

ALTER TABLE my_table 
ADD COLUMN description varchar(100) 
DEFAULT "B2"
COMMAND_I_D_WISH_TO_KNOW "A1";

changes my_table into

name   |   work       | description
-------------------------------------
bob    |  fireman     | "A1"
carl   |  teacher     | "A1"
alice  |  policeman   | "A1"

so that if afterwards I run the query

INSERT INTO my_table(name, work)
VALUES karen, developer;

my_tables becomes

name   |   work       | description
-------------------------------------
bob    |  fireman     | "A1"
carl   |  teacher     | "A1"
alice  |  policeman   | "A1"
karen  |  developer   | "B2"

Answer 1

Referencing the most recent docs, this operation can be done using two statements.

1. Adds the column with the old default value

ALTER TABLE my_table ADD COLUMN description varchar(100) DEFAULT 'A1';

2. Modifies the column to use a different default value

ALTER TABLE my_table ALTER COLUMN description SET DEFAULT 'B2'

A full reproducible sample has been included below:

CREATE TABLE my_table (
  "name" VARCHAR(5),
  "work" VARCHAR(9)
);

INSERT INTO my_table
  ("name", "work")
VALUES
  ('bob', 'fireman'),
  ('carl', 'teacher'),
  ('alice', 'policeman');

---

Query #1

select * from my_table;

name	work
bob	fireman
carl	teacher
alice	policeman

---

Query #2

ALTER TABLE my_table 
ADD COLUMN description varchar(100) 
DEFAULT 'A1';

There are no results to be displayed.

---

Query #3

select * from my_table;

name	work	description
bob	fireman	A1
carl	teacher	A1
alice	policeman	A1

---

Query #4

ALTER TABLE my_table 
ALTER COLUMN description SET DEFAULT 'B2';

There are no results to be displayed.

---

Query #5

INSERT INTO my_table("name", "work")
VALUES ('karen', 'developer');

There are no results to be displayed.

---

Query #6

select * from my_table;

name	work	description
bob	fireman	A1
carl	teacher	A1
alice	policeman	A1
karen	developer	B2

---

View working demo on DB Fiddle

Let me know if this works for you.