mysql例子

1、1. 建一张学生表 包含（ id ， name ， age ， sex ）

\2. 增加四条数据 

\3. 查询表中 sex 为男的数据 

\4. 删除 id =3 的数据， 

\5. 将 sex 为女的，修改为男



    create table students(id int,
                         name varchar(20),
                         age int,
                         sex varchar(4));
                         
    insert into students values(1, "小白", 18, "男"),
                            (2, "小张", 19, "男"),
                            (3, "小红", 20, "女"),
                            (4, "小泼", 21, "女");
                            
    select * from students where sex = "男";                        
    
    delete from students where id = 3;
    
    update students set sex = "男" where sex = "女";
    

 

统计出每个年级分别有多少人

统计出每个年级age大于18的人数

统计出一年级的人数

统计出一年级age大于18的人数

create table students(id int primary key auto_increment,
                     name varchar(4),
                     age int,
                     grade varchar(20))auto_increment = 201512940;
                     
insert into students (name, age, grade) values
( '刘一', 16, '一年级'),
( '陈二', 17, '二年级'),
( '张三', 18, '一年级'),
( '李四', 19, '三年级'),
( '王五', 20, '一年级'),
( '赵六', 21, '二年级'),
( '孙七', 22, '一年级'),
( '周八', 23, '一年级'),
( '吴九', 23, '三年级'),
( '郑十', 23, '一年级');

select grade, count(name) from students group by grade;

select grade, count(name) from students where age > 18 group by grade;

select count(name) from students where grade = "一年级";

select count(name) from students where grade = "一年级" and age > 18;