联合查询2_CDA答疑社区

类型转换88->88.00

select cast(88 as decimal(4,2))

输出字符串 '11'

select cast(1 as char(1))+'1'

创建视图,视图中存储的是select语句,而不是结果集数据，常用的复杂语句存到一个视图里

create view ab
as
select * from student --视图的内容是一个select语句

select * from ab

删除视图

drop view ab

子查询，在班级表里的各个班级的学生信息

select * from student
where cid in (select distinct cid from class)

exists关键字,大数据效率更高些

select * from student
where exists
(select * from class where student.cid=class.cid)

将性别作两列,分别显示年龄

select name 姓名,
max(case when gender='1' then age else 0 end) 男,
avg(case when gender='0' then age else 0 end) 女
from student
group by name