求教一条SQL语句

求教一条SQL语句 在线等啊
string strk = "select top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme join MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc";


为什么我这条SQL语句读取出来的数据 同一条数据会显示两次呢？ 要怎样修改呢？ 求教大神

------解决方案--------------------------------------------------------
select distinct ....
------解决方案--------------------------------------------------------
select 
top 20 MobileTheme.ThemeName ,
MobileTheme .DownLoadUrl,
MobileThemePreviews .Preview 
from MobileTheme join MobileThemePreviews 
on MobileThemePreviews.MobileThemeId=MobileTheme.Id group by MobileTheme.ThemeName ,
MobileTheme .DownLoadUrl,
MobileThemePreviews .Preview order by Downloads desc
加个 group by
------解决方案--------------------------------------------------------

探讨

distinct 要整表扫描，性能不如group by 过滤

------解决方案--------------------------------------------------------
join 改成 left join