MariaDB 10.6

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create table test (a int, b int);
insert into test values(1,1);
insert into test values(1,2);
create table test2 (a int, b int,
UNIQUE KEY uktest2 (a,b));
INSERT INTO test2
  (a,b)
  SELECT a,b
    FROM test b
   WHERE a=1
      ON DUPLICATE KEY UPDATE a = b.a,
                              b=b.b;

Records: 2  Duplicates: 0  Warnings: 0


select * from test;
select * from test2;


a
b


1
1

1
2




a
b


1
1

1
2


insert into test values(1,3);
insert into test values(1,1);
INSERT INTO test2
  (a,b)
  SELECT a,b
    FROM test b
   WHERE a=1
      ON DUPLICATE KEY UPDATE a = b.a,
                              b=b.b;

Records: 4  Duplicates: 0  Warnings: 0


select * from test;
select * from test2;


a
b


1
1

1
2

1
3

1
1




a
b


1
1

1
2

1
3