Postgres 13

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CREATE TABLE users ("plan_id" INTEGER, "date" DATE, "new_date" DATE);

INSERT INTO users ("plan_id", "date", "new_date") VALUES
  (1, '2022-05-21', null),
  (2, '2022-04-15', null);

CREATE TABLE plans ("id" INTEGER, "months" INTEGER);

INSERT INTO plans ("id", "months") VALUES
  (1, '5'),
  (2, '1');

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 2

UPDATE users AS u
SET new_date = u.date + INTERVAL '1 month' * p.months
FROM plans AS p
WHERE p.id = u.plan_id;

UPDATE 2

SELECT * FROM users;

plan_id
date
new_date

1
2022-05-21
2022-10-21

2
2022-04-15
2022-05-15

SELECT 2