MySQL 8.0

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select version();


version()


8.0.30


CREATE TABLE sample_table (
  Id INT,
  AccountId VARCHAR(5)
)
INSERT INTO sample_table
VALUES
(201,  'a-123'),
(NULL, 'a-123'),
(NULL, 'a-123'),
(202,  'a-234'),
(NULL, 'a-234'),
(NULL, 'a-234'),
(203,  'a-345');
SELECT * FROM sample_table


Id
AccountId


201
a-123

null
a-123

null
a-123

202
a-234

null
a-234

null
a-234

203
a-345


SELECT
  CASE WHEN Id IS NULL THEN 
          (SELECT Id 
           FROM sample_table 
           WHERE AccountId = a.AccountId
           LIMIT 1)
       ELSE Id
  END AS Id,
  AccountId
FROM sample_table a
ORDER BY Id ASC


Id
AccountId


201
a-123

201
a-123

201
a-123

202
a-234

202
a-234

202
a-234

203
a-345


SELECT b.Id,
       a.AccountId
FROM sample_table a
CROSS JOIN sample_table b
  ON a.AccountId = b.AccountId AND b.Id IS NOT NULL
ORDER BY b.Id ASC


Id
AccountId


201
a-123

201
a-123

201
a-123

202
a-234

202
a-234

202
a-234

203
a-345


UPDATE sample_table a
JOIN (SELECT c.Id,
             b.AccountId
      FROM sample_table b
          CROSS JOIN sample_table c
               ON b.AccountId = c.AccountId AND c.Id IS NOT NULL
      )  AS d ON a.AccountId=d.AccountId  
SET a.id = d.id
SELECT * FROM sample_table


Id
AccountId


201
a-123

201
a-123

201
a-123

202
a-234

202
a-234

202
a-234

203
a-345

Id	AccountId
201	a-123
null	a-123
null	a-123
202	a-234
null	a-234
null	a-234
203	a-345