alter table table_name
add [column] column_name column_definition [first|after existing_column];

alter table table_name
add [column] column_name column_definition [first|after existing_column],
add [column] column_name column_definition [first|after existing_column],
...;

create database test;
use test;

create table if not exists vendor (
	id int auto_increment primary key,
  name varchar(255)
);

alter table vendor
add column phone varchar(15) after name;

alter table vendor
add column vendor_group int not null;

insert into vendor(name, phone, vendor_group)
values('IBM', '(408)-298-2987', 1);
insert into vendor(name, phone, vendor_group)
values('Microsoft', '(408)-298-2988', 1);

alter table vendor
add column email varchar(100) not null,
add column hourly_rate decimal(10, 2) not null;

select id, name, phone, vendor_group, email, hourly_rate
from vendor;

+----+-----------+----------------+--------------+-------+-------------+
| id | name      | phone          | vendor_group | email | hourly_rate |
+----+-----------+----------------+--------------+-------+-------------+
|  1 | IBM       | (408)-298-2987 |            1 |       |        0.00 |
|  2 | Microsoft | (408)-298-2988 |            1 |       |        0.00 |
+----+-----------+----------------+--------------+-------+-------------+
2 rows in set (0.00 sec)

alter table vendor
add column vendor_group int not null;

select if(count(*) = 1, 'Exist', 'Not Exist') as result
from information_schema.columns
where table_schema = 'test'
	and table_name = 'vendor'
	and column_name = 'phone';

+--------+
| result |
+--------+
| Exist  |
+--------+
1 row in set (0.00 sec)

alter table 表名 add column 列名 類型 after 已有列名 comment '注釋信息';
alter table students add column class varchar(40) after student_name   comment '學(xué)生班級';

alter table 表名 drop column 列名;
alter table students drop column grade;