1. 显示系统时间(注：日期+时间)

      1. select now();

    2. 查询员工号，姓名，工资，以及工资提高百分之20%后的结果（new salary）

      select employee_id,last_name,salary,salary*1.2 '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) 长度,substr(last_name,1,1) 首字符,last_name from employees ORDER BY 首字符;

    4. 做一个查询，产生下面的结果

    earns monthly but wants
    Dream Salary
    King earns 24000 monthly but wants 72000

    select concat(last_name,' earns ',salary,' monthly but wants ',salary*3) as "Dream Salary" from employees where salary=24000;