#.下面的语句是否可以执行成功
 SELECT last_name, first_name, salary AS sal FROM employees;

 #.下面的语句是否可以执行成功
 表达式全角和半角注意事项

 #.
 #.显示departments表结构，并查询其中全部数据
 DESC departments;
 SELECT * FROM departments;
 #.显示employees中全部的job_id(不能重复)
 SELECT DISTINCT job_id FROM employees;
 #.显示表employeed的全部列。各个列之间用逗号连接，列头显示成out_put:null＋会全部为null
  SELECT
   IFNULL (commission_pet, ) AS 奖金率
 FROM
   employees;

 SELECT
   CONCAT (
     'first_name',
     ',',
     'last_name',
     ',',
     'job_id',
     ',',
     IFNULL (commission_pet, )
   ) AS out_put
 FROM
   employees;