What is ALTER command? How we use it to Add a new column?

Question

I am using below statement but i got error.
ALTER TABLE XXXtable
ADD xxx_name varchar2(50) not null;

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

Answer 1

ALTER command is used to modify the structure of table. By using alter we can add, modify & delete columns. to change the data type or decrease the size of a column the column must be empty. For ALTER a table we need ALTER, INSERT & CREATE privileges.
Ex: ALTER table

ADD type(size);