SQL Server 2019 (Linux)

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create table t ( a int, b int, c int, d int) ;
alter table t add unique(b, c);
insert into t (a, b, c, d) values (1, 1, 1, 1), (2, 2, null, 2), (3, 2, null, null);
Msg 2627 Level 14 State 1 Line 1
Violation of UNIQUE KEY constraint 'UQ__t__A86D1816648685EC'. Cannot insert duplicate key in object 'dbo.t'. The duplicate key value is (2, <NULL>).
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.

select * from t;


a
b
c
d