mysql之select(一)

select

初始准备工作：

1、建木瓜库并选中

create database mugua;
use mugua;

2、创建商品表、栏目表、品牌表

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,
market_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
);

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

create table brand (
brand_id smallint primary key auto_increment,
brand_name varchar(30) not null default ''
);

3、分别把shop库(ECShop软件里面的库)的商品数据、栏目数据、品牌数据导入到木瓜库

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;

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

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

select5种子句：

where 条件查询

group by 分组

having 筛选

order by 排序

limit 限制结果条数

where

常用运算符:

<	小于
>	大于
!=或<>	不等于
<=	小于等于
>=	大于等于
in	在某集合内
between....and.....	在某范围内

in(值1,值2,....,值n)等于值1~值n任意一个都行。

between 值1 and 值2，表示在值1(包括)和值2(包括)之间。

例：查询第4个栏目或第5个栏目的商品。

select goods_id,cat_id,goods_name from goods where cat_id in(4,5);

例：取出商品本店价格在2000和3000之间的商品。

select goods_id,goods_name,shop_price from goods where shop_price between 2000 and 3000;

not或!	逻辑非
or或||	逻辑或
and或&&	逻辑与

例：想买3000-5000之间的商品，但不用between and。

select goods_id,goods_name,shop_price from goods where shop_price >= 3000 and shop_price <= 5000;

例：想买3000-5000之间的商品，或者500-1000之间的商品。

select goods_id,goods_name,shop_price from goods
where shop_price between 3000 and 5000
or shop_price between 3000 and 5000

select goods_id,goods_name,shop_price from goods
where shop_price >= 3000 and shop_price <= 5000
or shop_price >= 500 and shop_price <= 1000;

not的用法：

例：取出不属于第4,5个栏目的商品。

select goods_id,cat_id,goods_name from goods where cat_id <> 4 and cat_id <> 5;

select goods_id,cat_id,goods_name from goods where cat_id not in(4,5);

模糊查询

like→像

例：想查找’诺基亚‘开头的所有商品。

select goods_id,goods_name from goods
where goods_name like '诺基亚%';

例：取’诺基亚NXX’系列。

select goods_id,goods_name from goods where goods_name like '诺基亚N__';

注意：

‘%’----通配任意字符

‘_’----通配单个字符

group by

常用于统计场合

常与下列聚合函数一起使用：

max : 求最大

min : 求最小

sum : 求总和

avg : 求平均

count:求总行数

例：查出最贵的商品的价格。

select max(shop_price) from goods;

例：查出最大（最新）的商品价格。

select max(goods_id) from goods;

例：查出最便宜的商品价格。

select min(shop_price) from goods;

例：查出最旧(最小)的商品编号。

select min(goods_id) from goods;

例：查询该店所有商品的库存总量。

select sum(goods_number) from goods;

例：查询该店所有商品的平均价格。

select avg(shop_price) from goods;

例：查询该店一共有多少种商品。

select count(*) from goods;

例：查询每个栏目下面，最贵商品价格、最低商品价格、商品平均价格、商品库存量、商品种类。提示(5个聚合函数,sum,avg,max,min,count与group综合运用)。

select cat_id,max(shop_price) from goods group by cat_id;
select cat_id,min(shop_price) from goods group by cat_id;
select cat_id,avg(shop_price) from goods group by cat_id;
select cat_id,sum(goods_number) from goods group by cat_id;
select cat_id,count(*) from goods group by cat_id;

例：请查询出本店每个商品比市场价格低多少？（要把列名当成变量来看待！！！）

select goods_id,goods_name,market_price - shop_price from goods;

例：查询每个栏目下面积压的货款。

select cat_id,sum(shop_price * goods_number) from goods group by cat_id;

可以给列或计算结果取别名，用as

select cat_id,sum(shop_price * goods_number) as hk from goods group by cat_id;

例：查询出本店价格比市场价低多少钱，并且把低200元以上的商品选出来。

1、先做前半部分

select goods_id,goods_name,market_price - shop_price as sheng from goods;

2、再做后半部分

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

select goods_id,goods_name,market_price - shop_price as sheng from goods having sheng > 200;

例：同上题，只不过查第3个栏目下比市场价低200以上的商品。

select goods_id,cat_id,market_price - shop_price as sheng from goods
where cat_id = 3
having sheng > 200;

例：查询积压货款超过2W元的栏目，以及该栏目所积压的货款。

1、先做后半句

select cat_id,sum(shop_price * goods_number) as hk from goods
group by cat_id;

2、再用having完成前半句。

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

having与where异同点:

having与where类似，可筛选数据，where后的表达式怎么写，having就怎么写。

where针对表中的列发挥作用，查询数据。

having针对查询结果中的列发挥作用，筛选数据。

where对表起作用，having是对结果进行筛选。

练习：设有成绩表student，如下：



查询挂科两门及两门以上不及格同学的平均分。

解一：

1、查看每个人的平均分。

select name, avg(score) from student group by name;

2、查看每个人的挂科情况。

select name,score < 60 from student;

3、计算每个人的挂科科目数。

select name,sum(score < 60) from student group by name;

4、综合以上各步，得出

select name,sum(score < 60) as gk,avg(score) as pj from student group by name
having gk >= 2;

解法二：

如何用子查询查询挂科两门及两门以上不及格同学的平均分，where型和from型不限。

先把挂科2门及2门以上的同学找出来。

①

select name,count(*) as gks from student where score < 60 group by name having gks >= 2;

②

select name from
(select name,count(*) as gks from student where score < 60 group by name having gks >= 2) as temp;

③我们用where+from型子查询，查询挂科两门及两门以上不及格同学的平均分。

select name,avg(score) from student
where name
in (select name from
(select name,count(*) as gks from student where score < 60 group by name having gks >= 2) as temp) group by name;

order by

排序可以根据字段来排序，根据字段来排序可以升序排，也可以降序排。默认是升序排列(ASC)，降序排列(DESC)。

例：按照价格把第3个栏目下的商品由低到高(升序)排列。

select goods_id,cat_id,goods_name,shop_price from goods
where cat_id = 3
order by shop_price (asc);

例：按照价格把第3个栏目下的商品由高到低(降序)排列。

select goods_id,cat_id,goods_name,shop_price from goods
where cat_id = 3
order by shop_price desc;

order by可以按多字段排序，order by 列1 [desc/asc],列2 [desc/asc]。

例如：

select goods_id,cat_id,goods_name,shop_price from goods
where cat_id <> 3
order by cat_id,shop_price desc;

例：按发布时间(add_time)由早到晚排序。

select goods_id,goods_name,add_time from goods
order by add_time asc;

limit

在语句的最后，起到限制条目的作用。

limit [offset,][N]

offset:偏移量

N: 取出条目(数)

offset如果不写，则相当于 limit 0,N。

例：取价格第4到第6高的商品。

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

例：取价格最高的前3个商品。

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

例：取最贵的商品。

select goods_id,goods_name,shop_price from goods
order by shop_price desc
limit 1;

例：取最新的商品。

select goods_id,goods_name,shop_price from goods
order by add_time desc
limit 1;

注意：例、取出每个栏目下的最贵的商品。

错误方式一：

#原因：分组之后，再按每个组的第一个排序。
select goods_id,cat_id,goods_name,shop_price from goods
group by cat_id
order by shop_price desc;

错误方式二：

1、建立一张临时表。

create table g2 like goods;

2、向临时表中导入源表排好序的数据。

insert into g2
select * from goods
order by cat_id asc,shop_price desc;

3、查询。

select goods_id,cat_id,goods_name,shop_price from g2;

注意：清空表语法

truncate g2;

正确做法（可使用from子查询语句）：

select * from
(select goods_id,cat_id,goods_name,shop_price from goods
order by cat_id asc,shop_price desc) as tmp
group by cat_id;#按每个组的第一个排序

注意：5个子句是有顺序要求的：where,group by,having,order by,limit。

良好的理解模型：

where 表达式，把表达式放在行中，看表达式是否为真。列：理解成变量，可以运算。

取出结果：可以理解成一张临时表。

子查询

子查询：子查询就是在原有的查询语句中，嵌入新的查询，来得到我们想要的结果集。一般根据子查询的嵌入位置分为，where型子查询，from型子查询。

where 型子查询：把内层查询的结果作为外层查询的比较条件。

例：查询最新的商品(以id为最大为最新)。

select goods_id,goods_name from goods
order by goods_id desc
limit 1;

不让用order by：

select goods_id,goods_name from goods
where goods_id = (select max(goods_id) from goods);

例：用where子查询，取出每个栏目下的最新的商品(以id为最大为最新)。

select goods_id,cat_id,goods_name,shop_price from goods
where goods_id
in (select max(goods_id) from goods group by cat_id);

例：用where子查询，取出每个栏目下的最贵的商品。

select goods_id,cat_id,goods_name,shop_price from goods
where shop_price
in (select max(shop_price) from goods group by cat_id)

from 型子查询：把内层的查询结果当成临时表，供外层sql再次查询。

例：用from子查询，取出每个栏目下的最新的商品。

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

exists 型子查询：外层sql查询所查到的行代入内层sql查询，要使内层查询能够成立 。查询可以与in型子查询（？）互换，但效率要高。

例：查有商品的栏目。

select cat_id,cat_name from category
where exists
(select * from goods where goods.cat_id = category.cat_id);