Oracle: UNIQUE constraint

Unique constraints on columns that are nullable

create table tq84_uq (
   x   number   not null,
   y   varchar2(10) null,
   val number,
   unique(x,y)
);
 
insert into tq84_uq values (1,'one', 1);
insert into tq84_uq values (1, null, 0);
insert into tq84_uq values (2, null, 2);
 
-- ORA-00001: unique constraint .... violated
insert into tq84_uq values (2, null, 3);

Adding unique constraints

alter table T add                            unique(col[, col_2…]);
alter table T add constraint constraint_name unique(col[, col_2…]);

See also