How can I drop a “not null” constraint in Oracle when I don't know the name of the constraint?

Question

I have a database which has a NOT NULL constraint on a field, and I want to remove this constraint. The complicating factor is that this constraint has a system-defined nam

Answer 1

Something like that happened to me when I made copies of structures to temporary tables, so I removed the not null.

DECLARE
   CURSOR cur_temp_not_null IS
        SELECT table_name, constraint_name  FROM all_constraints WHERE table_name LIKE 'TEMP_%' AND  owner='myUSUARIO';

   V_sql VARCHAR2(200); 

BEGIN
  FOR c_not_null IN cur_temp_not_null
   LOOP
     v_sql :='ALTER TABLE ' || c_not_null.table_name || ' DROP CONSTRAINT '|| c_not_null.constraint_name;
     EXECUTE IMMEDIATE  v_sql;     
  END LOOP;
END;

Answer 2

I was facing the same problem trying to get around a custom check constraint that I needed to updated to allow different values. Problem is that ALL_CONSTRAINTS does't have a way to tell which column the constraint(s) are applied to. The way I managed to do it is by querying ALL_CONS_COLUMNS instead, then dropping each of the constraints by their name and recreate it.

select constraint_name from all_cons_columns where table_name = [TABLE_NAME] and column_name = [COLUMN_NAME];

Answer 3

Just remember, if the field you want to make nullable is part of a primary key, you can't. Primary Keys cannot have null fields.

Answer 4

Try:

alter table <your table> modify <column name> null;

Answer 5

alter table MYTABLE modify (MYCOLUMN null);

In Oracle, not null constraints are created automatically when not null is specified for a column. Likewise, they are dropped automatically when the column is changed to allow nulls.

Clarifying the revised question: This solution only applies to constraints created for "not null" columns. If you specify "Primary Key" or a check constraint in the column definition without naming it, you'll end up with a system-generated name for the constraint (and the index, for the primary key). In those cases, you'd need to know the name to drop it. The best advice there is to avoid the scenario by making sure you specify a name for all constraints other than "not null". If you find yourself in the situation where you need to drop one of these constraints generically, you'll probably need to resort to PL/SQL and the data-definition tables.

Answer 6

To discover any constraints used, use the code below:

-- Set the long data type for display purposes to 500000.

SET LONG 500000

-- Define a session scope variable.

VARIABLE output CLOB

-- Query the table definition through the <code>DBMS_METADATA</code> package.

SELECT dbms_metadata.get_ddl('TABLE','[Table Described]') INTO :output FROM dual;

This essentially shows a create statement for how the referenced table is made. By knowing how the table is created, you can see all of the table constraints.

Answer taken from Michael McLaughlin's blog: http://michaelmclaughlin.info/db1/lesson-5-querying-data/lab-5-querying-data/ From his Database Design I class.