select * from my_class
union -- 默认去重
select * from my_class;
select * from my_class
union all -- 不去重
select * from my_class;
select id,c_name,room from my_class
union all -- 不去重
select name,number,idfrom my_student;
-- 需求：男生升序，女生降序（年龄）(
select * from my_studentwhere sex='男' order by age asc limit 9999999)
union(
select *from my_studentwhere sex='女' order by agedesc limit 9999999);
-- 标量子查询
select * from my_studentwhere c_id=(select id from my_classwhere name='python1910');
-- 列子查询
select * from my_studentwhere c_idin (select id from my_class);
-- any some all
select * from my_studentwhere c_id =any (select id from my_class);
select * from my_studentwhere c_id =some (select id from my_class);
select * from my_studentwhere c_id =all (select id from my_class);
select * from my_studentwhere c_id !=any (select id from my_class);-- 所有结果（null除外）
select * from my_studentwhere c_id !=some (select id from my_class);-- 所有结果（null除外）
select * from my_studentwhere c_id !=all (select id from my_class);-- 1,2不等于345（null除外）
select * from my_studentwhere age=(select max(age)from my_student)and height=(select max(height)from my_student);
-- 行子查询
select * from my_studentwhere (age,height)= (select max(age),max(height)from my_student);
select *from my_studentorder by age desc,heightdesc limit 1;-- 可能查询结果不是预想的
-- 表子查询
select * from my_studentgroup by c_idorder by heightdesc;
-- 插入学生
insert into my_studentvalues (null,'bc20200007','小红','女',25,185,1);
-- 查找每个班身高最高的学生
select * from (select * from my_studentorder by heightdesc limit 9999999)as studentgroup by c_id;
--
select exists(select * from my_student);
select exists(select * from my_studentwhere id=100);
-- exists子查询
select * from my_studentwhere exists(select * from my_class);-- 判断括号里是否成立
select * from my_studentwhere exists(select * from my_classwhere id=3);
select * from my_studentwhere exists(select * from my_classwhere id=2);
-- 视图：单表+多表
create view my_v1as select * from my_student;
create view my_v2as select * from my_class;
create view my_v3as select * from my_studentas sleft join my_classas con s.c_id=c.id;-- 错误：id重复
-- 多表视图
create view my_v3as select s.*,c.name as c_name,c.room from my_studentas sjoin my_classas con s.c_id=c.id;
show tables;
-- 查看视图创建语句
show create view my_v3\G
-- 视图的使用
select * from my_v1;
select * from my_v2;
select * from my_v3;
-- 修改视图
alter view my_v1as select id,name,age,sex,height,c_idfrom my_student;
-- 删除视图
create view my_v4as select * from my_student;
drop view my_v4;
-- 多表视图插入数据 : 不能插入数据
insert into my_v3values (null,'bc20200008','张三丰','男',150,180,3,'python1910','a204');
-- 将学生表的学号字段设置成不能为空
alter table my_studentmodify number char(10)not null unique ;
-- 单表视图插入数据： 视图不包括所有不允许为空的字段
insert into my_v1values (null,'bc20200008','张三丰',150,'男',180,3);
-- 单表视图插入数据
insert into my_v2values (2,'python1903','a204');
-- 多表视图删除数据: 不能删除数据
delete from mydb.my_v3where id=1;
-- 单表视图删除数据
delete from my_v2where id=4;
-- 多表视图更新数据
update my_v3set c_id=4 where id=6;
-- 视图： age 字段限制更新
create view my_v4as select * from my_studentwhere age>30 with check option;
-- 表示视图的数据来源都是年龄大于30岁的，是由where age>30 决定的
-- with check option决定通过视力更新的时候，不能将已经得到的数据age>30的改成<30的
-- 将视图可以查看到的数据改成年龄小于30
update my_v4set age=29 where id=5;
-- 可以修改数据让视图可以查到： 可以改，但是没有效果
update my_v4set age=32 where id=3
