foreign key constraints revisited

8/8/2019 Foreign Key Constraints Revisited

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Foreign Key Constraints Revisited

Many times readers send questions about how to handle foreign key constraints,when the primary key constraints on the parent tables have been altered.

Some readers asked for scripts to disable and re-enable foreign key constraints on allof the tables in a schema, as full/partial schema refreshes across environments are aregular requirement in databases.

I have simulated this test case to explain the requirements:

create user jp identified by jp account unlockdefault tablespace users temporary tablespace tempquota unlimited on users;

create table jp.manager (mgr_num number,mgr_name varchar2(20));

alter table jp.manager add(constraint manager_pk primary key (mgr_num));

SQL> desc jp.managerName Null? Type----------------------------------------- -------- ----------------------------MGR_NUM NOT NULL NUMBERMGR_NAME VARCHAR2(20)

The primary key constraint manager_pk is enforcing a not null constraint on the jp.manager.mgr_num column.

SQL> insert into jp.manager values(1,'BABU');

1 row created.

SQL> commit;

Commit complete.

create table jp.employee(emp_num number,emp_name varchar2(20), mgr_num number);

alter table jp.employee add (constraint employee_fkforeign key (mgr_num) references jp.manager(mgr_num));

SQL> desc jp.employeeName Null? Type----------------------------------------- -------- ----------------------------EMP_NUM NUMBEREMP_NAME VARCHAR2(20)



MGR_NUM NUMBER

By default, a foreign key constraint does not enforce a not null constraint on the childtable's column, jp.employee.mgr_num.

What if the MGR_NUM column in JP.EMPLOYEE table is nullable?

SQL> insert into jp.employee values(1,'BAJU',2);insert into jp.employee values(1,'BAJU',2)*ERROR at line 1:ORA-02291: integrity constraint

(JP.WORKER_FK) violated - parent key not found

SQL> insert into jp.employee values(1,'BAJU',null);

1 row created.

The foreign key constraint employee_fk did not allow us to insert value 2 into the

jp.employee.mgr_num column, instead giving us the error "parent key not found".

However, the said foreign key constraint employee_fk allowed us to insert a nullvalue in the jp.employee.mgr_num column. How can this be possible?

As per RDBMS definition, the value of null is unknown or undefined. Null is not equalto anything. A null is not equal to another null.

As such, Oracle does not know how to check for a matching record in the parent table jp.manager.mgr_num column, that matches with the null value in the child in the

jp.employee.mgr_num column.

To safeguard our data from this scenario, please enable a not null constraint on the jp.employee.mgr_num column.

truncate table jp.employee;

alter table jp.employee modify(mgr_num number not null);

insert into jp.employee values(1,'BAJU',1);insert into jp.employee values(2,'BASU',1);commit;

Let us drop the foreign key constraint on jp.employee table.

alter table jp.employee drop constraint employee_fk;

alter table jp.manager drop constraint manager_pk;

Now, a primary key constraint on mgr_num and mgr_name colums is added on the jp.manager table.



alter table jp.manager add (constraint manager_pk primary key(mgr_num,mgr_name));

SQL> desc jp.managerName Null? Type

----------------------------------------- -------- ----------------------------MGR_NUM NOT NULL NUMBERMGR_NAME NOT NULL VARCHAR2(20)

Now the prikery key constraint manager_pk on mgr_num and mgr_name of jp.manager table is enforcing not null constraints on both mgr_num and mgr_namecolumns.

Now let's recreate the emplyee_fk foreign key constraint on jp.employee table.

SQL> alter table jp.employee add (constraint employee_fk foreign key (mgr_num) references jp.manager(mgr_num)); 2constraint employee_fk foreign key (mgr_num) references jp.manager(mgr_num))

*ERROR at line 2:ORA-02270: no matching unique or primary key for this column-list

Since there is no primary key constraint or unique constraint enabled on the column jp.manager.mgr_num column exclusively, the employee_fk foreign key constraintcould not be enabled on the jp.employee table.

Let's first add a unique constraint on the jp.manager.mgr_num column and later addthe primary key constraint on jp.manager.mgr_num and jp.manager.mgr_namecolumns.

alter table jp.manager drop constraint manager_pk;alter table jp.manager add (constraint manager_uk unique (mgr_num));

SQL> desc jp.managerName Null? Type----------------------------------------- -------- ----------------------------MGR_NUM NUMBERMGR_NAME VARCHAR2(20)

Only a primary key constraint enforces a not null constraint on the tables' columns,

a unique constraint does not enforce a not null constraint on the tables' columns.

To safeguard and validate the data, it is a good practice to enable a not nullconstraint on the columns, where a unique constraint is created.

Since the primary key constraint on mgr_num and mgr_name columns will enforcenot null constraints, our requirement is met.

alter table jp.manager add (constraint manager_pk primary key(mgr_num,mgr_name));



SQL> desc jp.managerName Null? Type----------------------------------------- -------- ----------------------------MGR_NUM NOT NULL NUMBER

MGR_NAME NOT NULL VARCHAR2(20)

Now we can add the employee_fk foreign key constraint on the jp.employee table.

alter table jp.employee add (constraint employee_fk foreign key(mgr_num) references jp.manager(mgr_num));

Now I want to truncate all the tables in JP's schema and refresh with import from theproduction db.

SQL> select count(1) from jp.manager;

COUNT(1)----------

1

SQL> select count(1) from jp.employee;

COUNT(1)----------

2

The following scripts disable all foreign key constraints and truncate the tables in aparticular schema and re-enable the foreign key constraints.

Please make the necessary changes as per your specific requirements.

set serverout on size 1000000declarev_str varchar2(1000);beginfor c1 in (select owner,table_name from dba_tables where owner='JP') loopbeginfor c2 in (select a.owner,a.table_name,a.constraint_name

from all_constraints a, all_constraints bwhere a.constraint_type = 'R'and a.r_constraint_name = b.constraint_nameand a.r_owner = c1.owner

and b.table_name = c1.table_name) loopv_str:='alter table '||c2.owner||'.'||c2.table_name||' disable constraint '

||c2.constraint_name;--dbms_output.put_line(v_str);execute immediate v_str;end loop;--execute immediate 'truncate table '||c1.owner||'.'||c1.table_name;exceptionwhen others then



dbms_output.put_line(c1.table_name||' '||sqlerrm);end;end loop;end;/PL/SQL procedure successfully completed.


COUNT(1)----------

0


COUNT(1)----------

0

SQL> select constraint_name,status from dba_constraints2 where owner='JP' and table_name='EMPLOYEE';

CONSTRAINT_NAME STATUS------------------------------ --------SYS_C0032318 ENABLEDEMPLOYEE_FK DISABLED

I will refresh the JP's schema tables.

insert into jp.manager values(1,'BABU');insert into jp.employee values(1,'BAJU',1);

insert into jp.employee values(2,'BASU',1);commit;

set serverout on size 1000000declarev_str varchar2(1000);beginfor c1 in (select owner,table_name from dba_tables where owner='JP') loopbeginfor c2 in (select a.owner,a.table_name,a.constraint_name

from all_constraints a, all_constraints bwhere a.constraint_type = 'R'

and a.r_constraint_name = b.constraint_nameand a.r_owner = c1.ownerand b.table_name = c1.table_name) loop

v_str:='alter table '||c2.owner||'.'||c2.table_name||' enable constraint '

||c2.constraint_name;--dbms_output.put_line(v_str);execute immediate v_str;end loop;exception



when others thendbms_output.put_line(c1.table_name||' '||sqlerrm);end;end loop;end;/

PL/SQL procedure successfully completed.

SQL> select constraint_name,status from dba_constraints2 where owner='JP' and table_name='EMPLOYEE';

CONSTRAINT_NAME STATUS------------------------------ --------SYS_C0032318 ENABLEDEMPLOYEE_FK ENABLED


COUNT(1)----------

1


COUNT(1)----------

2

Taking into consideration reader's views and requirements, I thought that I would

revisit my article "FINDING FOREIGN KEY CONSTRAINTS" and fill in the gaps. Hencethis article "FOREIGN KEY CONSTRAINTS REVISITED".

foreign key constraints revisited

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