MariaDB 10.5

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select version();

version()

10.5.17-MariaDB-1:10.5.17+maria~deb11

create table c (code varchar(13), maxversion tinyint default 0);
insert into c values ('123456789abcd',1), ('123456789abcd',0),('123456789abcd',0),('123456789abcd',0),('123456789abcd',0)

Records: 5  Duplicates: 0  Warnings: 0

insert into c values ('dcba987654321',1), ('dcba987654321',0), ('dcba987654321',0), ('dcba987654321',0), ('dcba987654321',0)

Records: 5  Duplicates: 0  Warnings: 0

SELECT * FROM c

code
maxversion

123456789abcd
1

123456789abcd
0

123456789abcd
0

123456789abcd
0

123456789abcd
0

dcba987654321
1

dcba987654321
0

dcba987654321
0

dcba987654321
0

dcba987654321
0

alter table c add c_maxversion varchar(13) as (if(maxversion, code, NULL)) unique 

Records: 10  Duplicates: 0  Warnings: 0

select * from c

code
maxversion
c_maxversion

123456789abcd
1
123456789abcd

123456789abcd
0
null

123456789abcd
0
null

123456789abcd
0
null

123456789abcd
0
null

dcba987654321
1
dcba987654321

dcba987654321
0
null

dcba987654321
0
null

dcba987654321
0
null

dcba987654321
0
null

insert into c(code, maxversion) values ('123456789abcd',1)

Duplicate entry '123456789abcd' for key 'c_maxversion'

insert into c(code,  maxversion) values ('dcba987654321',1)

Duplicate entry 'dcba987654321' for key 'c_maxversion'

insert into c(code, maxversion) values ('123456789abcd',0)
insert into c(code,  maxversion) values ('dcba987654321',0)

code	maxversion
123456789abcd	1
123456789abcd	0
123456789abcd	0
123456789abcd	0
123456789abcd	0
dcba987654321	1
dcba987654321	0
dcba987654321	0
dcba987654321	0
dcba987654321	0