Question

Let’s say I have a table parent with primary key id, and a table child with foreign key parent_id and a “boolean” column (constrained to a 0 or 1), let’s call it is_initial.

What I want to do is put a constraint on child so that for a particular value of parent_id, there can be only one row with is_initial = 1. There can be any number of rows with is_initial = 0.

Can this be done with a constraint? I prefer not to add a trigger.

Thanks.

Answer 1

Here you go, I believe I understand what you are looking for now

Note the change in the unique index:

create unique index childTable_initialIndex on childTable(
  case when is_initial = 1 then parent_id 
  else null
  end);

Ammended Code

create table childTable(parent_id number, child_id number primary key, is_initial number, somethingelse varchar2(50) );

create unique index childTable_initialIndex on childTable(
  case when is_initial = 1 then parent_id 
  else null
  end);


  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (1,1,0,'works');
1 rows inserted.  

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (1,1,0,'will not work if childId is pk');
SQL Error: ORA-00001: unique constraint (SYS_C0062138) violated
00001. 00000 -  "unique constraint (%s.%s) violated"
*Cause:    An UPDATE or INSERT statement attempted to insert a duplicate key.
           For Trusted Oracle configured in DBMS MAC mode, you may see
           this message if a duplicate entry exists at a different level.
*Action:   Either remove the unique restriction or do not insert the key  

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (1,2,1,'works3');
1 rows inserted.  

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (1,3,1,'should not work');
SQL Error: ORA-00001: unique constraint (CHILDTABLE_INITIALINDEX) violated
00001. 00000 -  "unique constraint (%s.%s) violated"
*Cause:    An UPDATE or INSERT statement attempted to insert a duplicate key.
           For Trusted Oracle configured in DBMS MAC mode, you may see
           this message if a duplicate entry exists at a different level.
*Action:   Either remove the unique restriction or do not insert the key.  

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (2,4,0,'works4');
1 rows inserted.

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (2,5,0,'works5');
1 rows inserted.

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (2,6,1,'works6');
1 rows inserted.

  insert into childTable(parent_id, child_id, is_initial,somethingelse) values (2,7,1,'should not work');
SQL Error: ORA-00001: unique constraint (CHILDTABLE_INITIALINDEX) violated
00001. 00000 -  "unique constraint (%s.%s) violated"
*Cause:    An UPDATE or INSERT statement attempted to insert a duplicate key.
           For Trusted Oracle configured in DBMS MAC mode, you may see
           this message if a duplicate entry exists at a different level.
*Action:   Either remove the unique restriction or do not insert the key.  
  --we should only see things that work
  select * from childTable
/ 

--this should not work, since works already has the 1/1 is_initial 1
update childTable 
   set somethingelse = 'Should not work!'
     , is_initial    = 1
 where somethingelse = 'works';

SQL Error: ORA-00001: unique constraint (CHILDTABLE_INITIALINDEX) violated
00001. 00000 -  "unique constraint (%s.%s) violated"
*Cause:    An UPDATE or INSERT statement attempted to insert a duplicate key.
           For Trusted Oracle configured in DBMS MAC mode, you may see
           this message if a duplicate entry exists at a different level.
*Action:   Either remove the unique restriction or do not insert the key.

Here are the results:

PARENT_ID CHILD_ID IS_INITIAL SOMETHINGELSE                                    
--------- -------- ---------- --------------------------------------------------
        1        1          0 works                                              
        1        2          1 works3                                             
        2        4          0 works4                                             
        2        5          0 works5                                             
        2        6          1 works6