获取当前文件上一条与下一条记录的原理是上一条的sql语句，从news表里按从大到小的顺序选择一条比当前ID小的新闻,下一条的sql语句，从news表里按从小到大的顺序选择一条比当前ID大的新闻。
如果ID是主键或者有索引，可以直接查找：
方法1：
- 1.select * from table_a where id = (select id from table_a where id < {$id} order by id desc limit 1);
- 2.select * from table_a where id = (select id from table_a where id > {$id} order by id asc limit 1);
方法2：
- 1.select * from table_a where id = (select max(id) from table_a where id < {$id});
- 2.select * from table_a where id = (select min(id) from table_a where id > {$id});