mysql数据库的条件查询

mysql> select gender from students group by gender;
+--------+
| gender |
+--------+
| NULL   |
| ?      |
| ?      |
+--------+
3 rows in set (0.00 sec)

mysql> select name, gender from students group by name, gender;
+----------+--------+
| name     | gender |
+----------+--------+
| lisi     | ?      |
| peiqi    | ?      |
| waang    | NULL   |
| zhangsan | ?      |
| 平二        | ?      |
+----------+--------+
5 rows in set (0.00 sec)

mysql>

-- 根据gender字段进行分组， 查询gender字段和分组的name字段信息
mysql> select gender,group_concat(name) from  students group by gender;
+--------+--------------------+
| gender | group_concat(name) |
+--------+--------------------+
| NULL   | waang              |
| ?      | peiqi,zhangsan     |
| ?      | lisi,平二             |
+--------+--------------------+
3 rows in set (0.00 sec)

-- 统计不同性别的人的平均年龄
mysql> select gender,avg(age) from students group by gender;
+--------+----------+
| gender | avg(age) |
+--------+----------+
| NULL   |  15.0000 |
| ?      |   9.0000 |
| ?      |  18.0000 |
+--------+----------+
3 rows in set (0.00 sec)

-- 统计不同性别的人的个数
mysql> select gender,count(*) from students group by gender;
+--------+----------+
| gender | count(*) |
+--------+----------+
| NULL   |        1 |
| ?      |        2 |
| ?      |        2 |
+--------+----------+
3 rows in set (0.00 sec)

mysql>

-- 根据gender字段进行分组，统计分组条数大于2的
mysql> select gender,count(*) from students group by gender having count(*)>=2;
+--------+----------+
| gender | count(*) |
+--------+----------+
| ?      |        2 |
| ?      |        2 |
+--------+----------+
2 rows in set (0.00 sec)

-- 根据gender字段进行分组，汇总总人数
mysql> select gender,count(*) from students group by gender with rollup;
+--------+----------+
| gender | count(*) |
+--------+----------+
| NULL   |        1 |
| ?      |        2 |
| ?      |        2 |
| NULL   |        5 |
+--------+----------+
4 rows in set (0.00 sec)

-- 根据gender字段进行分组，汇总所有人的年龄
mysql> select gender,group_concat(age) from students group by gender with rollup;
+--------+-------------------+
| gender | group_concat(age) |
+--------+-------------------+
| NULL   | 15                |
| ?      | 0,18              |
| ?      | 18,18             |
| NULL   | 15,0,18,18,18     |
+--------+-------------------+
4 rows in set (0.00 sec)

mysql>

-- 为cls_id字段添加外键约束
alter table students add foreign key(cls_id) references classes(id);

-- 创建学校表
create table school(
    id int not null primary key auto_increment, 
    name varchar(10)
);

-- 创建老师表
create table teacher(
    id int not null primary key auto_increment, 
    name varchar(10), 
    s_id int not null, 
    foreign key(s_id) references school(id)
);

-- 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 teacher;

-- 获取名称之后就可以根据名称来删除外键约束
alter table teacher drop foreign key 外键名;

-- 创建 "京东" 数据库
create database jing_dong charset=utf8;

-- 使用 "京东" 数据库
use jing_dong;

-- 创建一个商品goods数据表
create table goods(
    id int unsigned primary key auto_increment not null,
    name varchar(150) not null,
    cate_name varchar(40) not null,
    brand_name varchar(40) not null,
    price decimal(10,3) not null default 0,
    is_show bit not null default 1,
    is_saleoff bit not null default 0
);

-- 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 '超极本' 的商品名称、价格

 select name,price from goods where cate_name = '超级本';
--显示商品的分类

 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,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),min(price),avg(price),count(*) 
 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 id,name,price from goods 
 where price > (select round(avg(price),2) as avg_price from goods) 
 order by price desc;