How to add new foreign key column with default value to an existing table with data - sql

Question

I'm trying to achieve this:
Table A that has almost 100 records - to add a new column (eg: ID
with default value 0)
Creating a new table B for which ID is PK
I tried this:
added ID column to Table A as null
updated the old values as 0 for existing rows
created Table B with ID as PK
made ID column as not null in Table A
tried to add FK constraint to Table A
And got this error:
ORA-02298: cannot validate parent keys not found.
What am I missing here? Also, I cannot delete child records in Table A

Answer 1

According to this Ask TOM article : Constraint ENABLE/ DISABLE
You can enable or disable integrity constraints at the table level
using the CREATE TABLE or ALTER TABLE statement. You can also set
constraints to VALIDATE or NOVALIDATE, in any combination with ENABLE
or DISABLE
And from a related artice by Tim Hall: ORACLE-BASE
ENABLE VALIDATE is the same as ENABLE. The constraint is checked and is guaranteed to hold for all rows.
ENABLE NOVALIDATE means the constraint is checked for new or modified rows, but existing data may violate the constraint.
DISABLE NOVALIDATE is the same as DISABLE. The constraint is not checked so data may violate the constraint.
DISABLE VALIDATE means the constraint is not checked but disallows any modification of the constrained columns.
So, to answer your question
"How to add new foreign key column with default value to an existing
table with data"
you may use option 2 ONLY if you want that the constraint is checked, but it does not have to be
true for all rows. This allows existing rows to violate the constraint, while ensuring
that all new or modified rows are valid.
ALTER TABLE A ADD FOREIGN KEY (ID) REFERENCES B(ID) ENABLE NOVALIDATE;

Answer 2

Starting scenario - CHILD_TABLE with 100 rows and no link to PARENT_TABLE:
create table parent_table (id integer constraint parent_pk primary key);
create table child_table (somecolumn integer);
insert into child_table select rownum from dual connect by rownum <= 100;
Now we want to link CHILD_TABLE to PARENT_TABLE with the default parent set to 0.
Add a value to PARENT_TABLE so that CHILD_TABLE can refer to it:
insert into parent_table (id) values (0);
If you don't add this value, it's like having a VEHICLES table with the default manufacturer as 'HONDA', but not defining 'HONDA' in the MANUFACTURERS table. The whole point of foreign keys is to stop you doing that.
Now add a foreign key to CHILD_TABLE (default on null is new in Oracle 12.1, but the plain default also works, however only recent versions of Oracle apply the default when adding a new column - I forget which version exactly.)
alter table child_table
add parent_id integer
default on null 0
constraint child_parent_fk references parent_table ;
Check what we have in CHILD_TABLE now:
select * from child_table;
SOMECOLUMN PARENT_ID
---------- ---------
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
10 0
11 0
...
100 rows selected
Check the status of the new FK:
select fk.constraint_name, fkc.column_name, fk.status
, pk.table_name, fk.r_constraint_name
from user_constraints fk
join user_constraints pk
on pk.constraint_name = fk.r_constraint_name
and pk.owner = fk.r_owner
join user_cons_columns fkc
on fkc.table_name = fk.table_name
and fkc.constraint_name = fk.constraint_name
where fk.table_name = 'CHILD_TABLE'
and fk.constraint_type = 'R'
and pk.constraint_type = 'P';
CONSTRAINT_NAME COLUMN_NAME STATUS TABLE_NAME R_CONSTRAINT_NAME
------------------ ------------------- -------- ------------------- -----------------
CHILD_PARENT_FK PARENT_ID ENABLED PARENT_TABLE PARENT_PK