综述：两张表，一张顾客信息表customers，一张订单表orders
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
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)
);在表中任意插入3条记录,c_name为"Zhangsan","Lisi","Wangwu", c_city尽量 写"Beijing","Shanghai" ......
insert into customers values
(1,"Zhangsan",25,"M","Beijing",8000),
(2,"Lisi",30,"F","Shanghai",10000),
(3,"Wangwu",27,"M","Shenzhen",3000);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
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
);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
insert into orders values
(1,"iphone",5288),
(1,"ipad",3299),
(3,"mate9",3688),
(2,"iwatch",2222),
(2,"r11",4400);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;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");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; 选择工资c_salary最少的顾客的信息
select * from customers
where c_salary = (select min(c_salary) from customers);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);
删除外键限制
1、show create table orders;
2、alter table orders drop foreign key orders_ibfk_1;
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;增加customers主键限制c_id
alter table customers add primary key(c_id);