各种数据库取表前N条记录的写法：
1．Oracle
Select * from table where rowNum<=n
2.Informix
Select first n* from table
3.DB2
Select * row_number() over(order by col1 desc) as rownum where rownum<=n
DB2
Select column from table fetch first n rows only
4.SqlServer
Select top n* from table
5.sysbase
Select top n* from table
6.mysql
Select * from table limit n