SQL> --Unique Key
SQL>
SQL>
SQL> create table myTable (
2 a number unique,
3 b number
4 );
Table created.
SQL>
SQL> desc myTable;
Name Null? Type
----------------------------------------------------------------------------------------------------- -------- --------------------------------------------------------------------
A NUMBER
B NUMBER
SQL>
SQL> -- if a column is not explicitely defined as not null, nulls can be inserted multiple times:
SQL>
SQL> insert into myTable values (4, 5);
1 row created.
SQL> insert into myTable values (2, 1);
1 row created.
SQL> insert into myTable values (9, 8);
1 row created.
SQL> insert into myTable values (6, 9);
1 row created.
SQL> insert into myTable values (null,9);
1 row created.
SQL> insert into myTable values (null,9);
1 row created.
SQL>
SQL> select * from myTable;
A B
---------- ----------
4 5
2 1
9 8
6 9
9
9
6 rows selected.
SQL>
SQL> drop table myTable;
Table dropped.
SQL>