hibernate中使用fetch来决策性能方案

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