什么时候用子查询，或者连接查询
一般多个数据的对应用子查询，单一行的数据用连接
（若要查询每个学生分别学了什么课程 ，若要fetch=join、fetch=select）
则是这种情况
Hibernate:
select
student0_.sid as sid2_,
student0_.s_name as s2_2_
from
student student0_
Hibernate:
select
courses0_.sid as sid2_1_,
courses0_.cid as cid1_,
course1_.cid as cid0_0_,
course1_.c_name as c2_0_0_
from
student_course courses0_
inner join
course course1_
on courses0_.cid=course1_.cid
where
courses0_.sid=?
freedom:odijfoij
freedom:in2
Hibernate:
select
courses0_.sid as sid2_1_,
courses0_.cid as cid1_,
course1_.cid as cid0_0_,
course1_.c_name as c2_0_0_
from
student_course courses0_
inner join
course course1_
on courses0_.cid=course1_.cid
where
courses0_.sid=?
wer:odijfoij
Hibernate:
select
courses0_.sid as sid2_1_,
courses0_.cid as cid1_,
course1_.cid as cid0_0_,
course1_.c_name as c2_0_0_
from
student_course courses0_
inner join
course course1_
on courses0_.cid=course1_.cid
where
courses0_.sid=?
ert:odijfoij
Hibernate:
select
courses0_.sid as sid2_1_,
courses0_.cid as cid1_,
course1_.cid as cid0_0_,
course1_.c_name as c2_0_0_
from
student_course courses0_
inner join
course course1_
on courses0_.cid=course1_.cid
where
courses0_.sid=?
qqs:odijfoij
子查询的效率：
Hibernate:
select
student0_.sid as sid2_,
student0_.s_name as s2_2_
from
student student0_
Hibernate:
select
courses0_.sid as sid2_1_,
courses0_.cid as cid1_,
course1_.cid as cid0_0_,
course1_.c_name as c2_0_0_
from
student_course courses0_
inner join
course course1_
on courses0_.cid=course1_.cid
where
courses0_.sid in (
select
student0_.sid
from
student student0_
)
freedom:in2
freedom:odijfoij
wer:odijfoij
ert:odijfoij
qqs:odijfoij