MariaDB 10.2

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CREATE TABLE codes 
  ( product_id  INT NOT NULL,
    code INT NOT NULL,
    ucode BIT AS (CASE WHEN code  = 11 THEN b'1' ELSE NULL END) VIRTUAL, -- PERSISTENT,
    CONSTRAINT code_11_product_id_unique
        UNIQUE (ucode, product_id)
  ) ;
insert into codes
  (product_id, code)
values 
  (1, 11),
  (1, 222),
  (1, 222),
  (1, 333),
  (2, 11),
  (2, 222),
  (2, 222);
select * from codes;


product_id
code
ucode


1
11
1

1
222
null

1
222
null

1
333
null

2
11
1

2
222
null

2
222
null


insert into codes          -- should fail
  (product_id, code)
values 
  (2, 11) ;
Duplicate entry '\x01-2' for key 'code_11_product_id_unique'

select * from codes;


product_id
code
ucode


1
11
1

1
222
null

1
222
null

1
333
null

2
11
1

2
222
null

2
222
null