* Into a row-level trigger based on a table, the trigger body cannot read data from the same table and also we cannot perform DML operation on the same table.
* If we are trying this oracle server returns mutating error oracle-4091: table is mutating.
* This error is called a mutating error, and this trigger is called a mutating trigger, and the table is called a mutating table.
* Mutating errors are not occurred in statement-level trigger because through this statement-level trigger when we are performing DML operations automatically data committed into the database, whereas in the row-level trigger when we are performing transaction data is not committed and also again we are reading this data from the same table then only mutating errors is occurred.
