Primary Keys Guarantee Uniqueness? Think Again.

When you create a table with a primary key or a unique constraint, Oracle automatically creates a unique index, to ensure that the column does not contain a duplicate value on that column; or so you have been told. It must be true because that is the fundamental tenet of an Oracle database, or for that matter, any database.

Well, the other day I was checking a table. There is a primary key on the column PriKey. Here are the rows:

Select PriKey from TableName;

PriKey
------
1
1

I got two rows with the same value. The table does have a primary key on this column and it is enforced. I can test it by inserting another record with the same value - “1”:

SQL> insert into TableName values (1,…)

It errors with ORA-00001: unique constraint violated error. The question is: why there are two rows with duplicate values in a column that has an enforced primary key, and it refuses to accept the very value that is already violated the primary key? It could be a great interview question, test your mettle; or just entertaining. Read on for the answer.