刘道成 mysql 学习笔记2

仿ecshop建库

create database mugua charset utf8;

use mugua;

create table goods(
goods_id int primary key auto_increment,
cat_id smallint not null default 0,
goods_sn char(15) not null default '',
goods_name varchar(30) not null default '',
click_count mediumint unsigned not null default 0,
brand_id smallint not null default 0,
goods_number smallint not null default 0,
marcket_price decimal(7,2) not null default 0.00,
shop_price decimal(7,2) not null default 0.00,
add_time int unsigned not null default 0 
)charset utf8;

从一个数据库导数据到另一个数据库的方法
会碰到中文出现乱码   先运行这个语句 set names gbk
insert into mugua.goods
select
goods_id,cat_id,goods_sn,goods_name,click_count,
brand_id,goods_number,market_price,shop_price,add_time
from shop.goods;


create table category(
cat_id smallint primary key auto_increment,
cat_name varchar(30) not null default '',
parent_id smallint not null default 0
)charset utf8;

insert into mugua.category
select
cat_id,cat_name,parent_id
from shop.category;

create table brand(

brand_id smallint primary key auto_increment,
brand_name varchar(30) not null default''
)charset utf8;

insert into mugua.brand
select
brand_id,brand_name
from shop.brand;

（一）where  表达式在哪一（几）行成立  就把那一（几）行取出来
  select * from goods where cat_id=3;
  select * from goods where cat_id !=3;
等价于
 select * from goods where cat_id <>3;
 select * from goods where cat_id>3;
 select * from goods where cat_id<3;
 select * from goods where cat_id>=3;
 select * from goods where cat_id<=3;
 select * from goods where cat_id  in(3,4);
 select * from goods where cat_id between 3 and 5;
 select * from goods where cat_id >3 or cat_id <=2;
 select * from goods where cat_id>=3 and cat_id<=6;

（二）group by  分组查询
   group by 与max  min  sum  avg  count连用
1 查出最贵商品价格
select max(shop_price) from goods;
2 查出最便宜商品价格
select min(shop_price) from goods;
3 查出商品总量
select sum(goods_number) from goods;
4 查出所有商品平均价格
select avg(shop_price) from goods;
5,查出所有商品总类
select count(goods_id) from goods;

#查询每个栏目下  要用到group by
1 查出最贵商品价格
select cat_id,max(shop_price) from goods group by cat_id;

2 查出最便宜商品价格
select cat_id,min(shop_price) from goods group by cat_id;
3 查出商品总量
select cat_id,sum(goods_number) from goods group by cat_id;
4 查出所有商品平均价格
select cat_id,avg(shop_price) from goods group by cat_id;
5,查出所有商品总类
select cat_id,count(goods_id) from goods group by cat_id;

#把列名当变量就可以进行运算了  可以写出更加高效的sql语句

#查询每个栏目下积压的货款
select cat_id,sum(shop_price*goods_number) from goods group by cat_id;
#查询出每个商品比市场价低多少
select goods_id,goods_name,marcket_price-shop_price as cha from goods;

（三）having查询
where 发挥作用的时间在查表结果之前  
having 是筛选  是对查询结果的进一步选择
1，查询比市场价省200元以上得物品价格以及所省的钱

select goods_name,goods_id,marcket_price-shop_price as sheng from goods having sheng>200;
如果写成 
select goods_name,goods_id,marcket_price-shop_price as sheng from goods where sheng>200;
是错误的

当然也可以写成

select goods_name,goods_id,marcket_price-shop_price as sheng from goods where marcket_price-shop_price>200;

只不错计算了两次  不如having 好


2，查询积压货款超过 20000元货款的栏目 以及该货款

select cat_id,sum(shop_price*goods_number) as jiya from goods group by cat_id having jiya>20000;

更多资源  http://www.taobtbw.com/
（四）order by 和limit查询
1，按价格由高到低排序
select goods_name,goods_id,shop_price from goods order by shop_price desc; 
2，按发布时间由早到晚排序
select goods_id,goods_name,add_time from goods order by add_time;

3，按栏目由低到高排序，栏目内部按价格由高到低排序

select goods_id,goods_name,cat_id,shop_price from goods order by cat_id asc,shop_price desc;

limit 的用法  limit在语句的最后  起到限制的作用
limit[offset]N
offset :偏移量
N;去处的条目数量
offset不写则相当于 limit 0，N
是从0 开始算的  而不是1

4,取出价格最高的前三名商品

select goods_id,goods_name,shop_price from goods order by shop_price desc limit 0,3;

5,取出点击量前三名到前5名的商品
select goods_id,goods_name,click_count from goods order by click_count desc limit 2,3;

思考题  ：取出每个栏目下最贵的商品
错误1：select goods_id,goods_name,cat_id,max(shop_price) from goods group by cat_id  错误原因是用了聚合函数  而其他栏目以第一次遇到的为准

错误2： select goods_id,goods_name,cat_id,shop_price from goods group by cat_id order by shop_price desc;这只是对分组后的排序 而不是对每一个栏目的排序

错误3：select goods_id,goods_name,cat_id,shop_price from goods order by shop_price desc group by cat_id ;   语法错误   因为查询字句是有顺序性的
顺序如下  where ， group by， having ，  order  by ，limit

正确：select * from (select goods_id,goods_name,cat_id,shop_price from goods order by cat_id asc,shop_price desc ) as temp group by  cat_id;

(四) 
（1）where from exists 子查询
1查询出最新商品（以最大编号为准）
select goods_name,goods_id from goods where goods_id=(select max(goods_id) from goods);
2,查询出编号为19的商品的栏目名称(待定)
3，用where子查询出每个栏目下的最新产品（以id号最大为准）
select goods_name,goods_id,cat_id from goods where goods_id in (select max(goods_id) from goods group by cat_id );  这里后面不能再加其他字段  否则会出错
4,用from子查询查出每个栏目下最新的产品（以id号最大为准）
select * from (select goods_name,goods_id,cat_id from goods order by cat_id asc,goods_id desc) as temp group by cat_id;  
5,exists 子查询    在category表中查询有商品的栏目  （先从category表中找出cat_id 然后与goods逐行比较）
select cat_id,cat_name from category where exists (select * from goods where category.cat_id=goods.cat_id);

注意几个问题  
1：from 子查询是把from后的查询结果当成一张临时表  一定要为临时表创建一个名字  用as  + 名字  表示
2；from子查询中的* 指的是from表中的字段  而不是原表的字段。
3.where 子查询是把内层查询的结果当外层查询的比较条件
4.exists 查询是把外层的查询结果拿到内层，看内层是否成立

更多资源请访问   http://www.17xuexiba.com/