Sometimes you have table in your system that has only one record like Configuration, Setup, Settings tables.
You want to prevent users from inserting more than one record in this table, To do this you can use the below INDEX .
CREATE UNIQUE INDEX ONE_ROW_INDEX
ON TABLE_NAME (1);
If you try to insert more than one record in previous table you will get below exception
ORA-00001: unique constraint (SCHEMA.ONE_ROW_ALLOWED) violated
Mahmoud A. El-Sayed