1 1、1. 建一张学生表 包含（ id ， name ， age ， sex ）
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3 \2. 增加四条数据
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5 \3. 查询表中 sex 为男的数据
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7 \4. 删除 id =3 的数据，
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9 \5. 将 sex 为女的，修改为男
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11
12
13 create table students(id int,
14 name varchar(20),
15 age int,
16 sex varchar(4));
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18 insert into students values(1, "小白", 18, "男"),
19 (2, "小张", 19, "男"),
20 (3, "小红", 20, "女"),
21 (4, "小泼", 21, "女");
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23 select * from students where sex = "男";
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25 delete from students where id = 3;
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27 update students set sex = "男" where sex = "女";
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1 统计出每个年级分别有多少人
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3 统计出每个年级age大于18的人数
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5 统计出一年级的人数
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7 统计出一年级age大于18的人数
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9 create table students(id int primary key auto_increment,
10 name varchar(4),
11 age int,
12 grade varchar(20))auto_increment = 201512940;
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14 insert into students (name, age, grade) values
15 ( '刘一', 16, '一年级'),
16 ( '陈二', 17, '二年级'),
17 ( '张三', 18, '一年级'),
18 ( '李四', 19, '三年级'),
19 ( '王五', 20, '一年级'),
20 ( '赵六', 21, '二年级'),
21 ( '孙七', 22, '一年级'),
22 ( '周八', 23, '一年级'),
23 ( '吴九', 23, '三年级'),
24 ( '郑十', 23, '一年级');
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26 select grade, count(name) from students group by grade;
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28 select grade, count(name) from students where age > 18 group by grade;
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30 select count(name) from students where grade = "一年级";
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32 select count(name) from students where grade = "一年级" and age > 18;
