1.可以执行，从 employees表中查询了last_name , job_id , salary（AS了一个别名为sal）

2.可以执行，查询employees表中全部的东西

3.修改后：

select employee_id,last_name,
salary * 12 AS "ANNUAL SALARY"
from employees;

从employees表中查找mployee_id,last_name,salary * 12 AS "ANNUAL SALARY" 并且将salary*12后的值设置一个别名为ANNUAL SALARY并查询出来

4.显示表结构代码：

DESC departments

显示全部：

5.使用以下代码

SELECT DISTINCT
employees.job_id
FROM
employees

6.运用CONCAT可以实现每个列之间逗号隔开，最后显示成out_put

SELECT* FROM employees;
SELECT CONCAT(employee_id,",",first_name,",",last_name,",",email,",
",phone_number,",",job_id,",",salary,",",IFNULL(commission_pct,0),",",
IFNULL(manager_id,0),",",department_id,",",hiredate) out_put FROM employees;