1. #1.查询没有奖金，且工资小于18000的salary，last_name
    2. SELECT
    3.   salary,
    4.   last_name,
    5.   commission_pct
    6. FROM
    7.     employees
    8. WHERE
    9.     salary < 18000 AND commission_pct IS NULL;
    12. #2.查询employees表中，job_id不为‘IT’或者工资为12000的员工信息
    13. SELECT
    14.     *
    15. FROM
    16.     employees
    17. WHERE
    18.     job_id <> 'IT' OR salary <=> 12000;
    21. #3.查看部门departments表的结构。
    22. DESC departments;
    23. #4.查询部门departments表中涉及到了哪些位置编号
    24. SELECT  
    25.     DISTINCT location_id
    26. FROM
    27.     departments;
    30. #5.经典面试题：
    31. /*
    32. 试问：select * from employees; 和
    33. select * from employees where commission_pct like '%%' and last_name '%%';
    34. 结果是否一样？并说明原因
    35. 不一样
    36. 因为有些字段会出现null
    37. */