constraints in create table..

[Saurabh Sharma]

hi all,   do anybody explain me what is happening when i'm creating a table with the following constraints..   CREATE TABLE TAB1( ENUM NUMBER PRIMARY KEY CHECK(ENUM IS NULL), ENAME VARCHAR2(10) );   why i'm being able to create table with such contradicting constraints. both can't be true at the same time. i can't have null in aprimary key.   what's the significance of this DDL.   thanks in advance..   Saurabh Sharma