1. -- 操作符
    2. -- distinct 去重
    3. -- 查询grade表里面的具体有哪些科目
    4. select distinct subject  from grade;
    6. -- in 多个相同的值
    7. -- 查询学生名称是老子和孔子具体信息
    8. select * from student where name = '孔子' or name = '老子' or name = '朱佳'
    9. select * from student where name in ('孔子','老子','朱佳');
    11. -- not in 不包含这些数据
    12. select * from student where name not in ('孔子','老子','朱佳');
    14. -- GROUP BY 分组
    15. -- 按照科目去分组，然后查询每个组的总分
    16. select subject,sum(score) from grade group by subject;
    19. -- 聚合函数
    20. -- avg(具体的列名)
    21. -- 求grader表的平均分
    22. select avg(score) from grade;
    23. -- 求语文的平均分
    24. select avg(score) from grade where subject = '语文';
    26. -- count(列名|*)
    27. -- 查询考语文的有几个
    28. select count(*) from grade where  subject = '语文';
    30. -- min(列名)
    31. -- 查询语文的最小成绩是多少
    32. select min(score) from grade  where subject = '语文';
    34. -- max(列名)
    35. -- 查询语文最高分
    36. select max(score) from grade  where subject = '语文';
    37. -- 查询grade表最高分
    38. select max(score) from grade 
    41. -- sum(列名)
    42. -- 求数学的总分是多少
    43. select sum(score)  from grade  where subject = '数学';
    48. -- select 列名 from 表名称
    49. select name from student;
    50. select id,name from student;
    52. -- * 查询所有
    53. select * from student;
    55. -- 使用比较运算符
    56. -- 查询大于60分的数据
    57. select * from grade where score > 60;
    58. -- 查询 成绩不大于60
    59. select * from grade where score !> 60;
    60. select * from grade where score < 60;
    62. -- 查询成绩不等于99的数据
    63. select * from grade where score != 99;
    64. select * from grade where score <> 99;
    66. -- 条件筛选
    67. -- and
    68. --查询成绩大于90 并且 科目是语文
    69. select * from grade where score > 90 and subject = '语文'
    71. -- or 
    72. -- 查询姓名是孔子或者老子
    73. select * from student where name = '孔子' or name = '老子';
    75. -- not 姓名不是庄子的
    76. select * from student where name not in ('孔子','庄子') 
    78. -- between 范围查询
    79. -- 查询 成绩在90-95之间的
    80. select * from grade where score between 90 and 95;
    82. -- not between 不在这个范围
    83. -- 查询 成绩不在80-90之间的
    84. select * from grade where score not between 80 and 90;
    86. -- 模糊查询
    87. -- %任意字符
    88. -- 查询名字是以子结尾
    89. select * from student where name like '老%';
    90. -- 查询名字中间有马
    91. select * from student where name like '%马%';
    94. -- _ 单个字符
    95. -- 查询名字是x子
    96. select * from student where name like '_子';
    98. -- NULL
    99. -- 查询gerade表里面s_id为null 
    100. select * from grade where s_id = null; -- 错误写法
    101. -- null值不可以使用等于 is
    102. select * from grade where s_id is null;
    104. -- not null
    105. select * from grade where s_id is not null;
    107. -- top(数量名词) 前几条数据
    108. select top (3) * from student;
    110. -- 排序 order by 
    111. -- order by 字段名称 asc | desc
    112. -- asc 升序
    113. select * from grade order by score asc;
    114. -- desc 降序
    115. select * from grade  order by score desc;
    117. -- 查询成绩前五名的
    118. select top(5) * from grade order by score desc;