The structure of table can be modified using ALTER COMMAND.
It is used to add or delete column in the table.
It is also used to modify already existing column of a table to either change its data-type or to increase or decrease its width/size of the column.
It is also used rename the columns of the table.

Following restrictions should be taken care of while ALTER COMMAND:
Change the name of the table
Change the name of the columnDecrease the size of a column if table data exists.