综述：两张表，一张顾客信息表customers，一张订单表orders

1、创建一张顾客信息表customers，字段要求如下：

    c_id 类型为整型，设置为主键，并设置为自增长属性

    c_name 字符类型，变长，宽度为20

    c_age 微小整型，取值范围为0~255(无符号)

    c_sex 枚举类型，要求只能在('M','F')中选择一个值

    c_city 字符类型，变长，宽度为20

    c_salary 浮点类型，要求整数部分最大为10位，小数部分为2位

    在表中任意插入3条记录,c_name为"Zhangsan","Lisi","Wangwu", c_city尽量写"Beijing","Shanghai" ......    

    create table customers(

    c_id int primary key auto_increment,

    c_name varchar(20),

    c_age tinyint unsigned,

    c_sex enum("M","F"),

    c_city varchar(20),

    c_salary float(12,2)

    );

    insert into customers values

    (1,"Zhangsan",25,"M","Beijing",8000),

    (2,"Lisi",30,"F","Shanghai",10000),

    (3,"Wangwu",27,"M","Shenzhen",3000);

2、创建一张订单表orders，字段要求如下：

    o_id 整型

    o_name 字符类型，变长，宽度为30

    o_price 浮点类型，整数最大为10位，小数部分为2位

    设置此表中的o_id字段为customers表中c_id字段的外键,更新删除同步

    在表中任意插入5条记录(注意外键限制)

    o_name分别为"iphone","ipad","iwatch","mate9","r11",其他信息自己定

    create table orders(

    o_id int,

    o_name varchar(30),

    o_price float(12,2),

    foreign key(o_id) references customers(c_id)

    on delete cascade

    on update cascade

    );

    insert into orders values

    (1,"iphone",5288),

    (1,"ipad",3299),

    (3,"mate9",3688),

    (2,"iwatch",2222),

    (2,"r11",4400);

3、返回customers表中，工资大于4000元，或者年龄小于29岁，满足这样条件的前2条记录

    select * from customers where c_salary > 4000 or c_age < 29 limit 2;

4、把customers表中，年龄大于等于25岁，并且地址是北京或者上海，这样的人的工资上调15%

   update customers set c_salary=c_salary*1.15 where c_age >= 25 and c_city in ("Beijing","Shanghai");

5、把customers表中，城市为北京的顾客，按照工资降序排列，并且只返回结果中的第一条记录

   select * from customers where c_city="Beijing" order by c_salary desc limit 1;

6、选择工资salary最少的顾客的信息

   select * from customers where c_salary = (select min(c_salary) from customers);

7、找到工资大于5000的顾客都买过哪些产品的记录明细

   select * from orders where o_id in (select c_id from customers where c_salary > 5000);

8、删除外键限制

   1、show create table orders;

   2、alter table orders drop foreign key orders_ibfk_1;

9、删除customers主键限制

   1、删除自增长属性

    alter table customers modify c_id int;

   2、删除主键限制

    alter table customers drop primary key;

10、增加customers主键限制c_id

   alter table customers add primary key(c_id);

select user_id,count(*) as plNum from comment group by user_id order by plNum desc limit 10;