Question

Suppose I have a

table User(userid, usernama) and a
table Owner(ownerid, ownername) and a joined
table UserOwnerMapping(userid, ownerid, IS_MASTER)
which create a many-to-many between user and owner

The is_master default is ‘N’. A user can only has one master.

How can I used a constraint to force no more than one master
for a user?

I am using Oracle.

Answer 1

You can achieve the constraint using a unique function-based index:

create unique index idx on userownermapping 
    (case when is_master='Y' then userid end);

This will only index the userid when is_master = ‘Y’, so forces userid to be unique when is_master = ‘Y’ but not otherwise:

SQL> create table UserOwnerMapping (userid integer, ownerid integer, IS_MASTER varchar2(1));

Table created.

SQL> 
SQL> create unique index idx on userownermapping 
  2      (case when is_master='Y' then userid end);

Index created.

SQL>     
SQL> insert into UserOwnerMapping (userid, ownerid, IS_MASTER) values (1, 2, 'Y');

1 row created.

SQL> 
SQL> insert into UserOwnerMapping (userid, ownerid, IS_MASTER) values (1, 3, 'N');

1 row created.

SQL>     
SQL> insert into UserOwnerMapping (userid, ownerid, IS_MASTER) values (1, 4, 'N');

1 row created.

SQL> 
SQL> insert into UserOwnerMapping (userid, ownerid, IS_MASTER) values (1, 5, 'Y');
insert into UserOwnerMapping (userid, ownerid, IS_MASTER) values (1, 5, 'Y')
*
ERROR at line 1:
ORA-00001: unique constraint (MYSCHEMA.IDX) violated