MySQL 8.0

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CREATE TABLE test(
id INT,
AccountId varchar(10) );

insert into test values
(201,'a-123'),
(NULL,'a-123'),
(NULL,'a-123'),
(202,'a-234'),
(203,'a-345');


select * from test;


id
AccountId


201
a-123

null
a-123

null
a-123

202
a-234

203
a-345


update test t
inner join (select id,AccountId 
            from test 
            where id is not null 
           )  as t2 on t.AccountId=t2.AccountId  
set t.id=t2.id
where t.id is null
select * from test;


id
AccountId


201
a-123

201
a-123

201
a-123

202
a-234

203
a-345