正如您所评论的，您正在使用mysql 8，我将使用公共表表达式和 ROW_NUMBER() 创建按顺序编号的行。
然后，我可以将它们连接回正在更新的表，并相互连接，以便将新的凭证id分配给没有凭证id的客户。
我也会用这个词 voucher ，因为您似乎互换使用了代金券和优惠券。

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
    ON c.row_id = v.row_id
SET
  customer.voucher_id = v.voucher_id
;

工作演示：https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=29875e64b3de3559ceea20d85d222f9b