MySQL 8.0

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CREATE TABLE voucher (
  voucher_id    INT,
  voucher_code  VARCHAR(32)
);

INSERT INTO
  voucher
VALUES
  (0, 'used_voucher'),   -- A voucher that has already been used
  (1, 'j2h3h2'),
  (2, 'hfd7s8'),
  (3, 'fsdfs72')
;

SELECT * FROM voucher;

Records: 4  Duplicates: 0  Warnings: 0




voucher_id
voucher_code


0
used_voucher

1
j2h3h2

2
hfd7s8

3
fsdfs72


CREATE TABLE customer (
  customer_id   INT,
  voucher_id    INT
);

INSERT INTO
  customer
VALUES
  (0,   0   ),   -- Customer who already has a voucher
  (100, NULL),
  (200, NULL),
  (300, NULL)
;

SELECT * FROM customer;

Records: 4  Duplicates: 0  Warnings: 0




customer_id
voucher_id


0
0

100
null

200
null

300
null


WITH
  renumbered_customer AS
(
  -- Find all rows in the customer table that do Not have an assigned voucher
  -- Number those rows sequentially from 1 upwards
  SELECT
    *,
    ROW_NUMBER() OVER (ORDER BY customer_id)   AS row_id
  FROM
    customer
  WHERE
    voucher_id IS NULL
),
  renumbered_voucher AS
(
  -- Find all rows in the voucher table that have not bee assigned to a customer
  -- Number those rows sequentially from 1 upwards
  SELECT
    *,
    ROW_NUMBER() OVER (ORDER BY voucher_id)   AS row_id
  FROM
    voucher
  WHERE
    NOT EXISTS (
      SELECT *
        FROM customer
       WHERE customer.voucher_id = voucher.voucher_id
    )
)
UPDATE
  customer
INNER JOIN
  renumbered_customer  c
    ON c.customer_id = customer.customer_id
INNER JOIN
  renumbered_voucher   v

Rows matched: 3  Changed: 3  Warnings: 0


SELECT
  c.customer_id, v.voucher_code
FROM
  customer c
LEFT JOIN
  voucher  v
    ON v.voucher_id = c.voucher_id


customer_id
voucher_code


0
used_voucher

100
j2h3h2

200
hfd7s8

300
fsdfs72