Question

子查询代替多表连接查询

select R.rname as 读者姓名,B.bname as 图书名称,P.pdate as 罚款日期,P.amount 罚款金额
from T_Book as B
inner join T_penalty as P on(B.bid=P.bid)
inner join T_Reader as R on(R.rid=P.rid)

我想用子查询代替多表连接查询，怎么改啊？

Answer 1

select (select rname from T_reader where rid=p.rid)as 读者姓名,
(select bname from t_book where bid=p.bid) as 图书名称,罚款日期,罚款金额
from T_penalty as p

子查询和多表查询一般可以相互转换。
两者都可以的情况下一般选择子查询实现需求。
hibernate需要制定实体之间的关系
Mybaits是为了给哪些不擅长写sql的人用的。
注解是一种框架的机制，可以少写一些配置文件而已。

Answer 2

可以这样写:
select (select rname from T_reader where rid=p.rid)as 读者姓名,
(select bname from t_book where bid=p.bid) as 图书名称,罚款日期,罚款金额
from T_penalty as p
但查询效率比连接要差很多.