ORA-01758: table must be empty to add mandatory (NOT NULL) column

While adding NOT NULL column into a table, the following error occurred:

SQL> Alter table TEST_TBL add NAME  CHAR(12) NOT NULL;
Alter table TEST_TBL add NAME  CHAR(12) NOT NULL
            *
ERROR at line 1:
ORA-01758: table must be empty to add mandatory (NOT NULL) column

The workarounds could be either of the following:

1) Default value is given:

SQL> Alter table TEST_TBL add NAME  CHAR(12) DEFAULT 'EMPTY' NOT NULL;

2) Remove NOT NULL:

SQL> Alter TEST_TBL add NAME CHAR(12);

3) Backup the table to another staging table, truncate the table and then add the NOT NULL column, insert the records back to the table from the staging table.