SQL & PL/SQL

Dear, Professionals.
I have 2 tables: P and C. C is a child table, P - parent. The FK from C to P is declared with "ON DELETE CASCADE" clause. Both tables are empty.
When I run

TRUNCATE TABLE P;

the output is:

Error starting at line 21 in command:
TRUNCATE TABLE P
Error report:
SQL Error: ORA-02266: tabela ma unikatowe/główne klucze do których odwołują się obce klucze
02266. 00000 -  "unique/primary keys in table referenced by enabled foreign keys"
*Cause:    An attempt was made to truncate a table with unique or
           primary keys referenced by foreign keys enabled in another table.
           Other operations not allowed are dropping/truncating a partition of a
           partitioned table or an ALTER TABLE EXCHANGE PARTITION.
*Action:   Before performing the above operations the table, disable the
           foreign key constraints in other tables. You can see what
           constraints are referencing a table by issuing the following
           command:
           SELECT * FROM USER_CONSTRAINTS WHERE TABLE_NAME = "tabnam";

How do you explain this?

Edited by: JackK on Dec 17, 2010 11:14 AM