Disable Constraint In Oracle

To disable constraint in the Oracle database, we have to use ALTER … DROP with constraint name. The constraint name can be a pre-defined name generated by the Oracle database or a user-defined name.

Syntax to disable the constraint in Oracle,

ALTER TABLE tablename
DISABLE CONSTRAINT constraint-name;

Since there can be only one primary key in a table, so we can also disable the primary key constraint in Oracle using the below syntax,

ALTER TABLE tablename
DISABLE PRIMARY KEY;

In the Oracle database, whenever we are creating constraint then the Oracle server internally automatically generates a unique identification number for identifying a constraint uniquely in the format of SYS_Cn, this is also called as pre-defined constraint name. Example:-

SQL> CREATE TABLE test (
       sno NUMBER(10) UNIQUE,
       name VARCHAR2(10) UNIQUE
     );
Table created.

SQL> INSERT INTO test VALUES(1, 'abc');
1 row created.

SQL> INSERT INTO test VALUES(1, 'pqr');
ORA-00001: unique constraint (SCOTT.SYS_C0014086) violated

SQL> INSERT INTO test VALUES(2, 'abc');
ORA-00001: unique constraint (SCOTT.SYS_C0014087) violated

Here SYS_C0014086 and SYS_C0014087 are the Oracle generated unique identification number of constraint applied on sno and name columns of the test table. We can get this information from user_cons_columns.

SQL> DESC user_cons_columns;
 Name              Null?    Type
 ----------------- -------- --------------
 OWNER             NOT NULL VARCHAR2(30)
 CONSTRAINT_NAME   NOT NULL VARCHAR2(30)
 TABLE_NAME        NOT NULL VARCHAR2(30)
 COLUMN_NAME                VARCHAR2(4000)
 POSITION                   NUMBER

SQL> SELECT column_name, constraint_name
     FROM user_cons_columns
     WHERE table_name = 'TEST';

COLUMN_NAME     CONSTRAINT_NAME
-----------     ---------------
SNO             SYS_C0014086
NAME            SYS_C0014087

Note that in the above query table name TEST is given in the capital letter. Oracle table name, column names are not case sensitive but records of a table are case sensitive, and here TEST is a record of user_cons_columns.

When Constraint is enabled at table level then the pre-defined constraint name will be same of those columns.

Example of disable the constraint having pre-defined name,

SQL> ALTER TABLE test
     DISABLE CONSTRAINT SYS_C0014086;
Table altered.

SQL> SELECT column_name, constraint_name
     FROM user_cons_columns
     WHERE table_name = 'TEST';

COLUMN_NAME     CONSTRAINT_NAME
-----------     ---------------
NAME            SYS_C0014087

While enabling/disabling/droping the constriants, pre-defined constraint name creates confusion for others and just by seeing pre-defined constraint name, we can’t identify which constraint is enabled on this column.

In place of pre-defined constraint name we can also assign our own name by using CONSTRAINT keyword by using following syntax. This is called user defined constraint name.

Syntax:-
CONSTRAINT user-definedName constriantType

Example:-

SQL> CREATE TABLE test1 (
       sno NUMBER(10) CONSTRAINT unique_sno UNIQUE,
       name VARCHAR2(10)
     );
Table created.

SQL> SELECT column_name, constraint_name
     FROM user_cons_columns
     WHERE table_name = 'TEST1';
COLUMN_NAME    CONSTRAINT_NAME
-------------- --------------
SNO            UNIQUE_SNO

Note:- User-defined constraint name must be unique within a user else Oracle gives an error.

Example of disable the constraint having user-defined name,

SQL> ALTER TABLE test1
     DISABLE CONSTRAINT unique_sno;
Table altered.

Now, constraint is disabled and we can insert any value into the column.

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