Oracle 18c

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create table test (a int, b int, c int);
create unique index idx_test on test(a, b, c);
insert into test values (1,1,null);
1 rows affected

insert into test values (1,1,null);
ORA-00001: unique constraint (FIDDLE_KYXGIADRYFGACBQEUUXE.IDX_TEST) violated

select *
from test;


A
B
C


1
1
null