数据库类

mysql查询第一条记录，select * from test limit 1; 

mysql查询第二条记录，select * from test limit 1,1;

mysql查询第2,3条记录，select * from test limit 1,2; 

mysql查询前2条记录，select * from test limit 0,2;

mysql查询最后一条记录，select * from test order by id desc limit1;