How can I drop a "not null" constraint in Oracle when I don't know the name of the constraint?_问答_开发者

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 name, and that constraint's name differs between the production server, integration server, and the various developer databases. Our current process is to check in change scripts, and an automated task executes the appropriate queries through sqlplus against the target database, so I'd prefer a solution that could just be sent straight into sqlplus.

On my own database, the SQL to drop this would be:

alter table MYTABLE drop constraint SYS_C0044566

I can see the constraint when I query the all_constraints view:

select * from all_constraints where table_name = 'MYTABLE'

but I am not sure how to work with the SEARCH_CONDITION's LONG data type or how best to dynamically delete the looked-up constraint even after I know its name.

So, how can I create a change script that can drop this constrai开发者_StackOverflownt based on what it is, rather than what its name is?

EDIT: @Allan's answer is a good one, but I am concerned (in my lack of Oracle expertise) that it may not be universally true that any constraint that might have a system-generated name will have associated with it a way to remove the constraint without having to know its name. Is it true that there will always be a way to avoid having to know a system-named constraint's name when logically dropping that constraint?

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.

Try:

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

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.

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.

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];

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;

If constraint on column STATUS was created without a name during creating a table, Oracle will assign a random name for it. Unfortunately, we cannot modify the constraint directly.

Steps involved of dropping unnamed constraint linked to column STATUS

Duplicate STATUS field into a new field STATUS2
Define CHECK constraints on STATUS2
Migrate data from STATUS into STATUS2
Drop STATUS column
Rename STATUS2 to STATUS

ALTER TABLE MY_TABLE ADD STATUS2 NVARCHAR2(10) DEFAULT 'OPEN'; ALTER TABLE MY_TABLE ADD CONSTRAINT MY_TABLE_CHECK_STATUS CHECK (STATUS2 IN ('OPEN', 'CLOSED')); UPDATE MY_TABLE SET STATUS2 = STATUS; ALTER TABLE MY_TABLE DROP COLUMN STATUS; ALTER TABLE MY_TABLE RENAME COLUMN STATUS2 TO STATUS;