SQL演练-建立关系表

建立关系表

创建goods表

create table goods(
id int unsigned primary key auto_increment not null,
name varchar(150) not null,
cate_name varchar(50),
brand_name varchar(50),
price int not null,
is_show bit not null default 1,
is_delete bit not null default 0
);

插入数据

-- 向goods表中插入数据
insert into goods values(0,'r510vc 15.6英寸笔记本','笔记本','华硕','3399',default,default); 
insert into goods values(0,'y400n 14.0英寸笔记本电脑','笔记本','联想','4999',default,default);
insert into goods values(0,'g150th 15.6英寸游戏本','游戏本','雷神','8499',default,default); 
insert into goods values(0,'x550cc 15.6英寸笔记本','笔记本','华硕','2799',default,default); 
insert into goods values(0,'x240 超极本','超级本','联想','4880',default,default); 
insert into goods values(0,'u330p 13.3英寸超极本','超级本','联想','4299',default,default); 
insert into goods values(0,'svp13226scb 触控超极本','超级本','索尼','7999',default,default); 
insert into goods values(0,'ipad mini 7.9英寸平板电脑','平板电脑','苹果','1998',default,default);
insert into goods values(0,'ipad air 9.7英寸平板电脑','平板电脑','苹果','3388',default,default); 
insert into goods values(0,'ipad mini 配备 retina 显示屏','平板电脑','苹果','2788',default,default); 
insert into goods values(0,'ideacentre c340 20英寸一体电脑 ','台式机','联想','3499',default,default); 
insert into goods values(0,'vostro 3800-r1206 台式电脑','台式机','戴尔','2899',default,default); 
insert into goods values(0,'imac me086ch/a 21.5英寸一体电脑','台式机','苹果','9188',default,default); 
insert into goods values(0,'at7-7414lp 台式电脑 linux ）','台式机','宏碁','3699',default,default); 
insert into goods values(0,'z220sff f4f06pa工作站','服务器/工作站','惠普','4288',default,default); 
insert into goods values(0,'poweredge ii服务器','服务器/工作站','戴尔','5388',default,default); 
insert into goods values(0,'mac pro专业级台式电脑','服务器/工作站','苹果','28888',default,default); 
insert into goods values(0,'hmz-t3w 头戴显示设备','笔记本配件','索尼','6999',default,default); 
insert into goods values(0,'商务双肩背包','笔记本配件','索尼','99',default,default); 
insert into goods values(0,'x3250 m4机架式服务器','服务器/工作站','ibm','6888',default,default); 
insert into goods values(0,'商务双肩背包','笔记本配件','索尼','99',default,default);

查询类型cate_name为 ‘超级本’ 的商品名称、价格

select name,price from goods where cate_name="超级本";--显示商品的种类
select cate_name from goods group by cate_name; --求所有电脑产品的平均价格,并且保留两位小数
select round(avg(price),2) as avg_price from goods; --显示每种商品的平均价格
select cate_name,round(avg(price),2) as avg_price from goods group by cate_name;--查询每种类型的商品中 最贵、最便宜、平均价、数量
select cate_name,max(price) from goods group by cate_name;
select cate_name,min(price) from goods group by cate_name;
select cate_name,avg(price) from goods group by cate_name;
select cate_name,count(*) from goods group by cate_name;
select cate_name,group_concat(name) from goods group by cate_name;--查询所有价格大于平均价格的商品，并且按价格降序排序
select * from goods where price>(select avg(price) from goods) order by price desc;--查询每种类型中最贵的电脑信息
select max(price) from goods;
select * from goods group by cate_name; 
select id,name,new_goods.cate_name,new_goods.max_price,price from goods inner join (select cate_name,max(price) as max_price from goods group by cate_name) as new_goods on goods.cate_name= new_goods.cate_name where goods.price=new_goods.max_price;

创建 “商品分类” 表

create table if not exists goods_cates(
id int unsigned primary key auto_increment,
name varchar(50) not null
);

查询goods表中商品的种类

select cate_name from goods;
select cate_name from goods group by cate_name

将分组结果写入到goods_cates数据表

insert into goods_cates(name) (select cate_name from goods group by cate_name);

通过goods_cates数据表来更新goods表

select * from goods as g inner join goods_cates as gc on g.cate_name=gc.name;
update (goods as g inner join goods_cates as gc on g.cate_name=gc.name) set g.cate_name=gc.id;

更新goods表的cate_name字段为cate_name_id

alter table goods change cate_name cate_name_id int unsigned not null;

通过create…select来创建”商品品牌表”并且同时写入记录,一步到位

create table goods_brands(
id int unsigned primary key auto_increment,
name varchar(40) not null) select brand_name as name from goods group by brand_name;

通过goods_brands数据表来更新goods数据表

select * from goods as g inner join goods_brands as gb on g.brand_name=gb.name;
update goods as g inner join goods_brands as gb on g.brand_name=gb.name set g.brand_name=gb.id;

更新goods表的brand_name字段为brand_name_id

alter table goods change brand_name brand_name_id int unsigned not null;

分别在 goods_cates 和 goods_brands表中插入记录

insert into goods_cates(name) values ('路由器'),('交换机'),('网卡');
insert into goods_brands(name) values ('海尔'),('清华同方'),('神舟');

在 goods 数据表中写入任意记录

insert into goods (name,cate_name_id,brand_name_id,price) values('LaserJet Pro P1606dn 黑白激光打印机', 12, 4,'1849');
insert into goods values(0,'LaserJet Pro P8888c 彩色激光打印机', 12, 4,'4998',default,default);

查询所有商品的详细信息 (通过内连接)

select g.id,g.name,c.name,b.name,g.price from goods as g
inner join goods_cates as c on g.cate_name_id=c.id
inner join goods_brands as b on g.brand_name_id=b.id;

查询所有商品的详细信息 (通过左连接)

select g.id,g.name,c.name,b.name,g.price from goods as g
left join goods_cates as c on g.cate_name_id=c.id
left join goods_brands as b on g.brand_name_id=b.id;

如何防止无效信息的插入,就是可以在插入前判断类型或者品牌名称是否存在呢? 可以使用之前讲过的外键来解决

外键约束:对数据的有效性进行验证
关键字: foreign key,只有 innodb数据库引擎支持外键约束
对于已经存在的数据表如何更新外键约束

-- 给brand_id 添加外键约束成功
alter table goods add foreign key (brand_name_id) references goods_brands(id);
-- 给cate_id 添加外键失败
-- 会出现1452错误
-- 错误原因:已经添加了一个不存在的cate_id值12,因此需要先删除(已添加)
alter table goods add foreign key (cate_name_id) references goods_cates(id);

如何在创建数据表的时候就设置外键约束呢?

create table goods(id int primary key auto_increment not null,name varchar(40) default '',price decimal(5,2),cate_name_id int unsigned,brand_name_id int unsigned,is_show bit default 1,is_saleoff bit default 0,foreign key(cate_name_id) references goods_cates(id),foreign key(brand_name_id) references goods_brands(id)
);

如何取消外键约束

-- 需要先获取外键约束名称,该名称系统会自动生成,可以通过查看表创建语句来获取名称
show create table goods;
CONSTRAINT `goods_ibfk_1` 
CONSTRAINT `goods_ibfk_2`
-- 获取名称之后就可以根据名称来删除外键约束
alter table goods drop foreign key 外键名称(goods_ibfk_1);

在实际开发中,很少会使用到外键约束,会极大的降低表更新的效率

创建 “顾客” 表

create table customer(id int unsigned auto_increment primary key not null,name varchar(30) not null,addr varchar(100),tel varchar(11) not null
);

创建 “订单” 表

create table orders(id int unsigned auto_increment primary key not null,order_date_time datetime not null,customer_id int unsigned,foreign key(customer_id) references customer(id)
);

创建 “订单详情” 表

create table order_detail(id int unsigned auto_increment primary key not null,order_id int unsigned not null,goods_id int unsigned not null,quantity tinyint unsigned not null,foreign key(order_id) references orders(id),foreign key(goods_id) references goods(id)
);