数据库-6-22

#行转列 id 和课程
select stu_id , '语文' as '课程', count_chinese as '分数' from student
union
select stu_id , '数学' as '课程', count_math as '分数' from student
union
select stu_id , '英语' as '课程', count_english as '分数' from student

#查看课程的及格数目
select
   sum(case when count_chinese>=60 then 1 else 0 end) 语文,
   sum(case when count_math>=60 then 1 else 0 end) 数学,
   sum(case when count_english>=60 then 1 else 0 end) 英语
from student

#统计stu_family表的各省人数 PostgreSQL和MySQL中可以这样写，原因是他们会先检查select语句
#select case
#   when stu_address='徐州' then '江苏'
#   when stu_address='无锡' then '江苏'
#   when stu_address='杭州' then '浙江'
#   when stu_address='乌鲁木齐' then '新疆'
#   when stu_address='哈尔滨' then '黑龙江'
#   when stu_address='广州' then '广东'
#   when stu_address='南京' then '江苏'
#   else '其它'
#   end 地区,
#   sum(stu_peoples) 人数
#from stu_family
#group by 地区;

#统计stu_family表的各省人数
select case
   when stu_address='徐州' then '江苏'
   when stu_address='无锡' then '江苏'
   when stu_address='杭州' then '浙江'
   when stu_address='乌鲁木齐' then '新疆'
   when stu_address='哈尔滨' then '黑龙江'
   when stu_address='广州' then '广东'
   when stu_address='南京' then '江苏'
   else '其它'
   end '地区',
   sum(stu_peoples) as '人数'
from stu_family
   group by case
       when stu_address='徐州' then '江苏'
       when stu_address='无锡' then '江苏'
       when stu_address='杭州' then '浙江'
       when stu_address='乌鲁木齐' then '新疆'
       when stu_address='哈尔滨' then '黑龙江'
       when stu_address='广州' then '广东'
       when stu_address='南京' then '江苏'
       else '其它' end;

#更改数据
update stu_family set stu_peoples=5 where stu_id=2

#增加字段
alter table stu_family add stu_peoples int

#插入数据
insert into stu_family(stu_number, stu_address) values
   (1,'徐州'),
   (2,'无锡'),
   (3,'杭州'),
   (4,'乌鲁木齐'),
   (5,'哈尔滨'),
   (6,'广州'),
   (7,'南京');

#添加一张表
create table stu_family
(
   stu_id int primary key auto_increment,#主键自增
   stu_address varchar(30),
   stu_phone varchar(15),
   stu_number int unique #唯一值
)

#设置自增
alter table student modify stu_id int auto_increment

#设置唯一值
alter table student add unique (stu_number)

#添加字段
alter table student add
(
stu_number int
)

#case判断单项
select stu_id '学号', count_math '数学',
   case
       when count_math>=99 then 'S'
       when count_math>=90 then 'A'
       when count_math>=75 then 'B'
       when count_math>=60 then 'C'
   else '不及格'
       end '等级'
from student

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