Postgres 14

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE tablea
    ("column_a" TEXT, "account_name" varchar(6))
;
    
INSERT INTO tablea
    ("column_a", "account_name")
VALUES
    (NULL, 'Worker'),
    (NULL, 'Boss')
;

CREATE TABLE

INSERT 0 2

CREATE TABLE tableb
    ("name" varchar(5), "account_name" varchar(8))
;
    
INSERT INTO tableb
    ("name", "account_name")
VALUES
    ('Bob', 'Worker'),
    ('Alice', 'Boss'),
    ('Tom', 'Worker'),
    ('Brad', 'Investor')
;

CREATE TABLE

INSERT 0 4

UPDATE tablea ta
SET "column_a" = (SELECT string_agg("name",',') FROM tableb WHERE "account_name" = ta."account_name")

UPDATE 2

SELECT * FROM tablea

column_a
account_name

Bob,Tom
Worker

Alice
Boss

SELECT 2