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+create database aa charset utf8;
+use aa;
+drop table `user`;
+drop table `order`;
+create table `user`(
+user_id int primary key,
+user_name varchar(10),
+user_tel varchar(20),
+ordertime date
+);
+desc `user`;
+insert into `user` values 
+(1,'张三',19865645691,'2023-3-13'),
+(2,'李四',12345675691,'2023-3-13'),
+(3,'王五',12895678901,'2023-3-13'),
+(4,'老六',12345678691,'2023-3-14'),
+(5,'小七',18945678031,null),
+(6,'老八',19345678561,'2023-3-16'),
+(7,'朱九',15845679911,null);
+create table `order`(
+order_id int,
+user_id int,
+foreign key(user_id) references `user`(user_id),
+type varchar(10),
+food varchar(10),
+number int,
+price int
+);
+insert into `order` values
+(1,1,'快餐','肯德基全家桶',1,50),
+(2,1,'快餐','可乐',4,20),
+(3,6,'饮料','柠檬水',2,16),
+(4,2,'米饭','鱼香肉丝盖饭',5,60),
+(5,2,'饮料','珍珠奶茶',3,22),
+(6,3,'粉面类','螺蛳粉',4,28),
+(7,4,'粉面类','凉拌面',1,10);
+-- 1.在用户表（user）新增一个性别字段user_sex，数据类型为char。
+alter table `user` add user_sex char;
+
+-- 2.由于顾客3退单，请把此订单删除。
+delete from `order` where user_id='3';
+
+-- 3.老八的电话号码换新了，请将他的号码更新为8888888888。
+update `user` set user_tel='8888888888' where user_name='老八';
+
+-- 4.查询order表里的最高金额和总金额。
+select
+ max(price) 最高金额,(select max(price * number)from `order`)总金额 
+from `order`;
+
+-- 5.查询每个顾客分别订购了几个类别。
+select 
+  user_id,type,count(type)
+from `order`
+group by user_id,type;
+
+-- 6.查询饮料的总金额
+select
+ sum(price * number)
+ from `order`
+where type='饮料';
+
+-- 7.查询订单表（order）里的订单号，类别（type），食物（food）.
+select
+ order_id,type,food
+from `order`;
+
+-- 8.查询没吃饭的人的所有信息
+select
+
+ * from `user` a left join `order` b on 
+    a.user_id=b.user_id
+   where a.user_id in (select distinct user_id from `order`)
+   group by a.user_id; 
+
+-- 9.查询姓老的人的全部信息及订单信息
+select
+
+ * from `user` a left join `order` b on 
+    a.user_id=b.user_id
+   where a.user_name like '老%';
+
+-- 10.查询总金额大于20的订单里最大值的用户信息。
+select
+
+ * from `user` a left join `order` b on 
+   a.user_id=b.user_id
+   where price * number in (select max(price * number) from `order`) and price * number>20;
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