求教一条SQL语句 在线等啊

[小刘哈哈]

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

[pz25925]

select distinct ....

[_老吴]

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

[_老吴]

distinct 要整表扫描，性能不如group by 过滤

[pz25925]

[Quote=引用 3 楼 的回复:]

distinct 要整表扫描，性能不如group by 过滤
[/Quote]

学习学习了.

[小刘哈哈]

[Quote=引用 2 楼 的回复:]
select
top 20 MobileTheme.ThemeName ,
MobileTheme .DownLoadUrl,
MobileThemePreviews .Preview
from MobileTheme join MobileThemePreviews
on MobileThemePreviews.MobileThemeId=MobileTheme.Id group……
[/Quote]

为什么 我group by 、distinct 加上去 都是报错呢？

string strk = "select top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme join MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc group by MobileTheme.ThemeName";

[风2013]

join 改成 left join

[叮咚叮咚叮叮咚]

string strk = "select top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme INNER JOIN MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc";

不知道这样是否可行

[糊涂浩浩]

你这两张join的表没有一个主表么，你直接join其实默认的是全连接，左表ID的会显示，右表ID的也会显示，当然是会显示两条数据了

[_老吴]

group by放在order by前面

[小刘哈哈]

还是 没解决 2、3楼的写法我这里报错 我是这样写的
2楼：string strk = "select distinct top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme join MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc";
3楼：string strk = "select top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme join MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc group by MobileTheme.ThemeName";

6、7楼的作法还是没改变 读取的数据还是 重复 多给点意见吧 大神

[_老吴]

string strk = "select top 20 MobileTheme.ThemeName ,MobileTheme .DownLoadUrl,MobileThemePreviews .Preview from MobileTheme join MobileThemePreviews on MobileThemePreviews.MobileThemeId=MobileTheme.Id order by Downloads desc group by MobileTheme.ThemeName";
你这样写不报语法错误啊？⊙﹏⊙b汗，真遇到高人了