Mohamed Houri’s Oracle Notes

October 8, 2013

On how to enforce uniqueness when unique constraint is not possible

Filed under: Oracle — hourim @ 3:21 pm

The following select gives a list of existing table that do not possess a unique constraint

select table_name
from user_tables u1
where not exists (select null
from  user_constraints u2
where u2.table_name      = u1.table_name
and   u2.constraint_type = 'U');

However, the above select doesn’t mean that there isn’t any other uniqueness enforcement  implemented without a dedicated unique constraint.

Recently, I have been confronted to a business rule to be implemented in a Data Vault model which specifies that each Satellite or Satellite of Link table should have a unique key enforcement satisfying the following rule

For each couple of (col1, col2) I could have at most a unique record for a not null dv_load_end_date column.

A picture being worth a thousand words let’s see this through a real example

SQL> create table t1
(t1_sk            number
,dv_load_date     date
,seq_nbr          number
,n1               number
,n2               number
,status           varchar2(10)
,identification   varchar2(30)
,dv_load_end_date date);

SQL> alter table t1 add constraint t1_pk primary key (t1_sk,dv_load_date, seq_nbr);

The unique business rule would be described as follows: for each couple (t1_sk, n2) I could have at most one record for a null dv_load_end_date column

Is this doable via a unique constraint? No it is not.

And here where function based index comes to the rescue

SQL> create unique index ind_t1_uk on t1
(case when dv_load_end_date is null then t1_sk else null end
,case when dv_load_end_date is null then n2 else null end);

And here how the inserts go

SQL> insert into t1 (t1_sk ,dv_load_date ,seq_nbr,n1,n2,dv_load_end_date)
              values(1, sysdate, 1, 1, 100, null);

1 row created.

SQL> insert into t1 (t1_sk ,dv_load_date ,seq_nbr,n1,n2,dv_load_end_date)
              values(1, sysdate, 1, 1, 100, sysdate);

1 row created.

SQL> insert into t1 (t1_sk ,dv_load_date ,seq_nbr,n1,n2,dv_load_end_date)
              values(1, sysdate, 1, 1, 100, null);

insert into t1 (t1_sk ,dv_load_date ,seq_nbr,n1,n2,dv_load_end_date)
ERROR at line 1:
ORA-00001: unique constraint (XXX.IND_T1_UK) violated

SQL> insert into t1 (t1_sk ,dv_load_date ,seq_nbr,n1,n2,dv_load_end_date)
              values(1, sysdate, 1, 1, 200, null);
1 row created.

This post is the second one in a set of small scripts (blog articles) I decided to collect in my blog for my documentation and reuse purposes


  1. Mohamed, in your table t1, column n1 is the primary key. Therefore you can only ever have record containing one value for this column throughout the entire table – irrespective of any other column values!

    For your idea to have any merit, you must make a different column the primary key.

    Comment by Martin Rose — October 8, 2013 @ 3:46 pm | Reply

  2. Martin,

    You are absolutely right.

    Why haven’t I taken my real life example and modeled it as it is? This is what I have finally done and updated the blog


    Comment by hourim — October 9, 2013 @ 7:01 am | Reply

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