Oracle: can I create a constraint that checks a value within a group

Question

Thanks in advance for your help with this. In Oracle, is it possible to create a constraint so that there can only be one 'leader' per group:

How can I create a rule that prevents an additional 'leader' from being inserted if one already exists in a group?

Answer 1

Does this make sense? A function-based unique index:

SQL> create table test (col1 varchar2(10), col2 varchar2(10), col3 varchar2(10));

Table created.

SQL> create unique index ui1t on test (case when col2 = 'leader' then col1||col2 end);

Index created.

Testing:

SQL> insert into test values ('group1', 'leader', 'joe');

1 row created.

SQL> insert into test values ('group1', null    , 'diane');

1 row created.

SQL> insert into test values ('group1', null    , 'john');

1 row created.

SQL> insert into test values ('group1', null    , 'diane');

1 row created.

SQL> insert into test values ('group1', 'leader', 'mike');   --> another LEADER for GROUP1
insert into test values ('group1', 'leader', 'mike')
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.UI1T) violated


SQL> insert into test values ('group1', 'leader', null);   --> another LEADER for GROUP1
insert into test values ('group1', 'leader', null)
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.UI1T) violated


SQL>