User_tab_cols - nullable column?

Question

Dear All,
check the workflow:
C:\>sqlplus scott/tiger
SQL*Plus: Release 10.2.0.1.0 - Production on Fri Apr 4 16:20:48 2008
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> CREATE TABLE TEST
2 (ID NUMBER(*,0))
3 /
Table created.
SQL>
SQL> ALTER TABLE TEST ADD (CONSTRAINT TEST_ID_NOT_NULL CHECK (ID IS NOT NULL) EN
ABLE)
2 /
Table altered.
SQL>
SQL> DESC TEST
Name Null? Type
ID NUMBER(38)
SQL>
SQL> -- Null? -> NULL
SQL>
SQL> SELECT TABLE_NAME
2 ,NULLABLE
3 FROM USER_TAB_COLS
4 WHERE TABLE_NAME = 'TEST'
5 /
TABLE_NAME N
TEST Y
SQL>
SQL> -- Nullable? Since when? I've declared a constraint on it. Ok, let's try:
SQL>
SQL> INSERT INTO TEST (ID) VALUES (NULL)
2 /
INSERT INTO TEST (ID) VALUES (NULL)
ERROR at line 1:
ORA-02290: check constraint (SCOTT.TEST_ID_NOT_NULL) violated
SQL>
SQL> -- True, I have a check constraint on the ID.
SQL> -- But why is the column nullable if I check the user_tab_cols?
SQL> -- I don't see the difference between forcing Oracle to add
SQL> -- constraints automatically at creation time
SQL> -- (create table test (id number not null)) and
SQL> -- adding constraints manually... It will use constraints
SQL> -- anyway...
SQL>
SQL> -- Don't forget to clean up!
SQL> DROP TABLE TEST CASCADE CONSTRAINTS
2 /
Table dropped.
SQL>
Any thoughts?
Thanks,
Franky

Answer 1

Franky,
Oracle has worked this way for as long as I can remember. Observe
UT1 > create table t1 (col1   varchar2(10)  not null);
Table created.
UT1 > desc t1
Name                                      Null?    Type
COL1                                      NOT NULL VARCHAR2(10)
UT1 > select search_condition from dba_constraints where table_name = 'T1';
SEARCH_CONDITION
"COL1" IS NOT NULL
UT1 > drop table t1
  2  /
Table dropped.
UT1 > create table t1 (col1 varchar2(10) );
Table created.
UT1 > desc t1
Name                                      Null?    Type
COL1                                               VARCHAR2(10)
  1* alter table t1 add (Constraint t1col1 check (col1 is not null))
UT1 > /
Table altered.
UT1 > desc t1
Name                                      Null?    Type
COL1                                               VARCHAR2(10)
  1* select search_condition from dba_constraints where table_name = 'T1'
UT1 > /
SEARCH_CONDITION
col1 is not nullUsing the "not null" keywords in the create table results in Oracle setting a switch as being turned on. It is this setting that describe reads for the table not null column. But in both cases dba_constraints will show a check constraint.
It is just the way it is. I suggest using the "not null" key words on the table create rather than setting explicit column level constraints for the columns.
I just noticed that in the first case that the column name is in double quotes in the search condition and is the second it is not.
HTH -- Mark D Powell --