Simple unique constraint question

Question

HI ALL,
I have created a table as below, and added unique key to both the col of the table.
CREATE TABLE TT( A NUMBER,B NUMBER);
ALTER TABLE TT ADD CONSTRAINT UNQ_CNT UNIQUE(A,B);
INSERT INTO TT VALUES (&A,&B);
Enter value for a: 1
Enter value for b: null
Enter value for a: 1
Enter value for b: null
ORA-00001: unique constraint (SCOTT.UNQ_CNT) violatedwhy I am getting the above err as Null cannot be equal to Null so the record
should be unique from other record.
Kindly help in understanding this concept.
Regards,
AAK.

Answer 1

AAK,
When you create UNIQUE constraint on multiple
NULLABLE columns, uniqueness is checked for NON-NULL
values. So in your case, when you try to insert
another row, ORACLE detects that column A already has
same value (1) and hence it gives error. You can find
more details in documentation.Two rows that contain nulls for one or more key columns and the same
combination of values for the other key columns violate the constraint.
Not for NON-NULL values only.
Khurram