text: #1. 显示系统时间(注: 日期+时间)
  select now();
  
  #2. 查询员工号、姓名、工资, 以及工资提高百分之20%之后的结果(new salary)
  select employee_id, last_name, salary, salary*1.2 as 'new salary'
  from employees;
  
  #3. 将员工的姓名按首字母排序, 并写出姓名的长度 (length)
  select length(last_name) as name_length, substr(last_name, 1, 1) as first_letter, last_name
  from employees
  order by first_letter;
  
  #4. 做一个查询, 产生下面的结果
  #<last_name> earsn <salary> monthly but wants <salary*3>
  #Dream salary
  #King earns 24000 monthly but wants 72000
  SELECT concat(last_name, ' earns ', salary, ' monethly but wants ', salary*3) AS "Dream Salary"
  FROM employees
  WHERE salary=24000;
  
  #5. 使用case-when, 按照下面的条件:
  #	job		grade
  #	AD_PRES	  A
  #	ST_MAN	  B
  #	IT_PROG	  C
  #   SA_REP	  D
  #	ST_CLERK  E
  SELECT last_name, job_id AS job, 
  CASE job_id
  WHEN 'AD_PRES' THEN 'A'
  WHEN 'ST_MAN' THEN 'B'
  WHEN 'IT_PROG' THEN 'C'
  WHEN 'SA_REP' THEN 'D'
  WHEN 'ST_CLERK' THEN 'E'
  END AS Grade
  FROM employees;