Ms sql server column add alter table

For making modifications in tables, first of all, make table authors as given below :
create table authors(
id int  identity(1,1)
,FirstName varchar(100)
,Age varchar(100)

Adding new column LastName  in above existing table authors
Syntax : ALTER TABLE table_name ADD new_column_name data_type 

Example :ALTER TABLE authors ADD  LastName varchar(100) null

Dropping  column  in above existing table authors
Syntax :  ALTER TABLE  table_name DROP COLUMN column_name 
Example :   ALTER TABLE authors DROP COLUMN  LastName 

Changing data type of column of existing table
Syntax :  ALTER TABLE  table_name ALTER COLUMN column_name  data_type 
Example : ALTER TABLE authors ALTER COLUMN Age int ;