SQL

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SQL

数据以表格的形式出现
每行为各种记录名称
每列为记录名称所对应的数据域
许多的行和列组成一张表单
若干的表组成database

Windows 下运行mysql

在命令行提示符下net start mysql57 开启mysql服务

SQL命令

limit 限制查询返回的条数
select * from table limit 1;
offset 指定select数据开始查询的数据偏移量
select * from table limit 1 offset 1;
主键和外键关联
方法1
select * from table1 t1
join table2 t2 on ti.id=t2.sid
join table3 t3 on t3.sid=t2.di
where t1.name=’’;
方法2
select * from table1 t1， table2 t2, table3 t3
where t1.name=’’ and t1.id=t2.sid and t3.sid=t2.id;
select * from table1 order by(-字段);从大到小，如果不加-，就是按从小到大
select * from table1 id in(1, 2); 搜索id是1，2的数
foreign key(g_id) references tb_grade(id) 创建外键的命令
default null 默认为空

聚合函数

group by 字段按字段将记录分组
order by(-字段) 从大到小，如果不加-，就是按从小到大
count() 计数
sum() 求和
max() 找最大
min() 找最小
avg() 求平均

SQL应用

create database tb_test1 default charset utf8;
use tb_test1;
create table tb_grade(
id int auto_increment,
g_name varchar(30)  not null,
primary key(id)
 );
create table tb_student(
id int auto_increment,
s_name varchar(10) not null,
s_tel varchar(11),
g_id int default null,
primary key(id),
foreign key(g_id) references tb_grade(id)
);
insert into tb_grade(g_name) values('python');
insert into tb_grade(g_name) values('java');
insert into tb_grade(g_name) values('php');
insert into tb_grade(g_name) values('c');
insert into tb_grade(g_name) values('c++');
# 查看表
desc tb_student;
insert into tb_student (s_name, s_tel, g_id) values('李佩', '13441242541'
,1);
insert into tb_student(s_name, s_tel, g_id) values('李四', '15192776631',
2);
insert into tb_student (s_name, s_tel, g_id) values('王菲', '15784322678'
,3);
insert into tb_student(s_name, s_tel, g_id) values('张三', '19999991234',
4);
select * from tb_student s join tb_grade g on g.id=s.g_id where g.g_name=
'python';
# 下面这种方法也是关联主键外键的方法，但不推荐使用
select * from tb_student as s, tb_grade as g where g.id=s.g_id;
insert into tb_student(s_name, s_tel, g_id) values('学生1', '12345678901'
,1);
insert into tb_student(s_name, s_tel, g_id) values('学生2', '12445678901'
,1);
insert into tb_student(s_name, s_tel, g_id) values('学生3', '13445678901'
,2);
insert into tb_student(s_name, s_tel, g_id) values('学生4', '13445688901'
,2);
insert into tb_student(s_name, s_tel, g_id) values('学生5', '13445688902'
,3);
insert into tb_student(s_name, s_tel, g_id) values('学生6', '13445608902'
,3);
# 按字段g_id将记录分组，并求每个组的人数
select count(*) as '学生个数', g.g_name as '班级名称' from tb_student s join tb_grade g
on s.g_id=g.id group by(g.id);
# 求和
select sum(s.id) as '学生个数', g.g_name from tb_student s join tb_grade
g on s.g_id=g.id group by(g.id);
alter table tb_student add s_grade varchar(5);
#删除字段
alter table tb_student drop s_grade;
alter table tb_student change s_grade s_price int;
alter table tb_student add s_grades int;
update tb_student set s_grades=70 where id=1;
update tb_student set s_grades=80 where id=2;
update tb_student set s_grades=90 where id=3;
update tb_student set s_grades=96 where id=4;
update tb_student set s_grades=96 where id=5;
update tb_student set s_grades=60 where id=6;
update tb_student set s_grades=60 where id=7;
update tb_student set s_grades=60 where id=8;
update tb_student set s_grades=60 where id=9;
update tb_student set s_grades=60 where id=10;
#找最大值
select max(s_grades) from tb_student;
#找最小值
select min(s_grades) from tb_student;
#求平均值
select avg(s_grades) from tb_student;

"""

1. 创建学生表Student，有字段姓名s_name, 家庭地址s_addr, 性别s_sex, 电话s_tel, 
语文成绩s_philology, 数学成绩s_math,主键id，外键g_id(关键班级表)
2. 创建班级表Grade，有字段 班级名称g_name, 班级描述g_desc, 班级创建时间g_create_time, 
主键id
3. 创建学生扩展表StudentInfo, 有字段 家长姓名i_name, 家长电话i_phone,
主键id, 外键i_id(关联学生表)

习题：
1.插入班级表数据
2.插入学生表数据
3.插入学生扩展表数据

4.查询每个班级下有多少学生
5.查询每个班级下学生的语文成绩最高的学生，一个学生
6.查询每个班级下学生的数学成绩最低的学生，前三个学生

7.统计每个班级下语文成绩的平均成绩，以及总和
8.查询每个班级下语文成绩大于60分的学生信息
"""

"""
#创建数据库
create database test2 charset=utf8;
use test2;
#创建班级表
create table tb_grade (
id int auto_increment,
g_name varchar(30) not null,
g_desc varchar(30),
g_create_time date,
primary key(id)
);
#创建学生表
create table tb_Student(
id int auto_increment,
s_name varchar(20) not null,
s_sex varchar(5) not null,
s_addr varchar(50),
s_tel varchar(11),
s_philology int,
s_math int,
g_id int,
primary key(id),
foreign key(g_id) references tb_grade(id)
);
#创建学生扩展表
create table tb_studentinfo(
id int auto_increment,
i_name varchar(20),
i_phone varchar(11),
i_id int,
primary key(id),
foreign key(i_id) references tb_student(id)
);
#插入班级表数据
insert into tb_grade(g_name, g_desc, g_create_time) values('python','stud
y python', '2018-3-1');
insert into tb_grade(g_name, g_desc, g_create_time) values('java','study
java', '2018-3-26');
insert into tb_grade(g_name, g_desc, g_create_time) values('php', 'study
php', '2018-4-1');
insert into tb_grade(g_name, g_desc, g_create_time) values('HTML5', 'stud
y HTML5', '2018-4-26');
#插入学生表数据
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('刘备', '男', '蜀国', '12345678901', '100', '60', 1);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('关羽', '男', '蜀国', '12345678902', '80', '80', 1);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('张飞', '男', '蜀国', '12345678903', '59', '89', 1);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('孙权', '男', '吴国', '12345678910', '98', '75', 2);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('小乔', '女', '吴国', '12345678911', '99', '59', 2);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('周瑜', '男', '吴国', '12345678912', '99', '99', 2);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('曹操', '男', '魏国', '12345678920', '100', '89', 3);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('张辽', '男', '魏国', '12345678921', '59', '89', 3);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
g_id) values('王大锤', '男', '中国', '12345678999', '35', '59', 4);
#插入学生扩展表
insert into tb_studentinfo(i_name, i_phone, i_id) values('刘母', '1234567
8901', 1);
insert into tb_studentinfo(i_name, i_phone, i_id) values('刘父', '1234567
8909', 1);
insert into tb_studentinfo(i_name, i_phone, i_id) values('关父', '1234567
8908', 2);
insert into tb_studentinfo(i_name, i_phone, i_id) values('关母', '1234567
8907', 2);
insert into tb_studentinfo(i_name, i_phone, i_id) values('张母', '1234567
8906', 3);
insert into tb_studentinfo(i_name, i_phone, i_id) values('张父', '1234567
8905', 3);
insert into tb_studentinfo(i_name, i_phone, i_id) values('孙父', '1234567
8915', 4);
insert into tb_studentinfo(i_name, i_phone, i_id) values('孙母', '1234567
8916', 4);
insert into tb_studentinfo(i_name, i_phone, i_id) values('乔父', '1234567
8917', 5);
insert into tb_studentinfo(i_name, i_phone, i_id) values('乔母', '1234567
8918', 5);
insert into tb_studentinfo(i_name, i_phone, i_id) values('周母', '1234567
8919', 6);
insert into tb_studentinfo(i_name, i_phone, i_id) values('周父', '1234567
8914', 6);
insert into tb_studentinfo(i_name, i_phone, i_id) values('曹父', '1234567
8929', 7);
insert into tb_studentinfo(i_name, i_phone, i_id) values('曹母', '1234567
8928', 7);
insert into tb_studentinfo(i_name, i_phone, i_id) values('张母', '1234567
8927', 8);
insert into tb_studentinfo(i_name, i_phone, i_id) values('张父', '1234567
8926', 8);
insert into tb_studentinfo(i_name, i_phone, i_id) values('王父', '1234567
8998', 9);
insert into tb_studentinfo(i_name, i_phone, i_id) values('王母', '1234567
8997', 9);
insert into tb_student(s_name, s_sex, s_addr, s_tel, s_philology, s_math,
 g_id) values('赵云', '男', '蜀国', '12345678100', 99, 99, 1);
insert into tb_studentinfo(i_name, i_phone, i_id) values('赵父', '1234567
1010', 10);
#查询每个班级下有多少个学生
select count(*) as '学生人数', g_name as '班级名称' from tb_student s joi
n tb_grade g on s.g_id=g.id group by(g_name);
#查询每个班级下学生的语文成绩最高的学生，一个学生
select * from (select max(s_philology) as c, g_id from tb_student s1 grou
p by s1.g_id) t1 join tb_student t2 on t2.s_philology=t1.c join tb_grade g on g.
id=t2.g_id and t1.g_id=t2.g_id;
查询每个班级下学生的数学成绩最低的学生，前三个学生
select * from tb_student s where 2>=(select count(*) from tb_student 
s1 where s.s_math > s1.s_math and s1.g_id=s.g_id) order by s.g_id, s.s_math;
"""