表名：firend(好友表)
结构设计
Id int not null primary key
self_id int null
firend_id int null
索引设计：self_id，firend_id两个字段份分别有独立索引
SQL语句功能：类似人人网查找可能认识的好友(间隔两个好友)
Select t. firend_id from (Select c. firend_id from firend a, firend b, firend c
Where a.self_id = :arg_id and a. firend_id = b. self _id and
b. firend_id = c. self _id ) as t
where t. firend_id not in (Select firend_id where self_id = :arg_id)
注：:arg_id为检索参数，表示自己的用户ID