Question

和ibatis及oracle有关的问题！

刚进公司上班，看到公司的代码很多这么写
<select id="selectRoleIdsByUserId" parameterClass="long" resultClass="long">
select t1.role_id
from Sys_User_Role t1
where t1.user_id = #userId#
and exists (select 1
from sys_role t2
where t2.role_id = t1.role_id
and t2.status = '1')
</select>

及

<select id="selectRoleIdsByUserIdOnRolegroup" parameterClass="long" resultClass="long">
select t1.role_Id
from Sys_rolegroup_role t1
where exists (select 1
from sys_rolegroup t2
where t2.rolegroup_id = t1.rolegroup_id
and t2.status = '1'
and exists (select 1
from sys_oper_rolegroup t3
where t3.rolegroup_id = t2.rolegroup_id
and t3.oper_id = #operId#))
and exists (select 1
from sys_role t4
where t4.role_id = t1.role_id
and t4.status = '1')
</select>

我想问select 1 from有作用及好处啊？还有那个exist。不太理解，可以帮忙解释详细一点吗？就拿第一个举例。本人菜鸟一个！

Answer 1

select 1 from 作用就是按照查询WHERE子句的条件查出符合条件的结果集，exist是判断 主表中是否存在与子查询相同的某条记录。

Answer 2

SELECT 1 判断“子”表。好处说不上吧，应该是某人的习惯

exist就是存在记录则真

Answer 3

SELECT 1 等价于 SELECT *