MariaDB 10.2

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create table tableX
( columnA int,
  CONSTRAINT columnA_UQ UNIQUE (columnA)
);
insert into tableX (columnA)
values (1), (2), (null) ;
insert into tableX (columnA)
values (1) ;
Duplicate entry '1' for key 'columnA_UQ'

ALTER TABLE tableX
  ADD columnA_single_null int AS (CASE WHEN columnA IS NULL THEN 1 END), 
  ADD CONSTRAINT Single_null_UQ 
    UNIQUE (columnA_single_null) ; 
insert into tableX (columnA)
values (1), (2), (null) ;
Duplicate entry '1' for key 'columnA_UQ'

select columnA from tableX ;


columnA


null

1

2


insert into tableX (columnA)
values (null) ;
Duplicate entry '1' for key 'Single_null_UQ'

select columnA from tableX ;


columnA


null

1

2

columnA
null
1
2

columnA
null
1
2