1. 接口和抽象类的区别
http://kb.cnblogs.com/page/42159/
2. DataSet和DataReader的区别
http://www.cnblogs.com/jiahaohk/articles/1546864.html
3.索引
http://www.2cto.com/database/201105/91731.html
http://kb.cnblogs.com/page/44125/
4. sql: 行转列，递归，删除重复行，group by having, left righ inner, 分页