1. 创建表dept1

  1. create table dept1(
  2.     id int(7),
  3.     name varchar(25)
  4. );

2. 将表departments中的数据插入新表dept2中

create table dept2 select department_id,department_name from departments;
select * from dept2;

3. 创建表emp5

create table emp5(
  id int(7),
  first_name varchar(25),
  last_name varchar(25),
  dept_id int (7)
);

4. 将列Last_name的长度增加到50

alter table emp5 modify column last_name varchar(50);
#查看
desc emp5 last_name

5. 根据表employees创建employees2

create table employees2 like employees;
select * from employees2;

6. 删除表emp5

drop table if exists emp5;

7. 将表employees2重命名为emp5

alter table employees2 rename to emp5;

8.在表dept和emp5中添加新列test_column，并检查所作的操作

alter table emp5 add column test_column int;

9.直接删除表emp5中的列 dept_id

DESC emp5;
ALTER TABLE emp5 DROP COLUMN test_column;