LINQ To Entity同時使用OrderBy與Distinct的小技巧
在LINQ To Entity中若先使用OrderBy方法對資料進行排序後，再用Distinct方法去除重複資料，轉成TSQL命令時OrderBy敘述會被取消，導致排序效果失效。
using (CTSEntities context = new CTSEntities())
{
ObjectQuery<conference_data> oqConferenceData = context.conference_data;
var result = (from p in oqConferenceData
orderby p.Times ascending
select p.Times).Distinct();
}
使用SQL Profiler錄製TSQL敘述結果如下：
SELECT
[Distinct1].[Times] AS [Times]
FROM ( SELECT DISTINCT
[Extent1].[Times] AS [Times]
FROM [dbo].[conference_data] AS [Extent1]
) AS [Distinct1]
問題癥結點出在這兩個方法的呼叫順序，只要改成先做Distinct再做OrderBy即可解決無法排序的問題，修改後的LINQ to Entity語法如下：
using (CTSEntities context = new CTSEntities())
{
ObjectQuery<conference_data> oqConferenceData = context.conference_data;
var result = (from p in oqConferenceData
select p.Times).Distinct().OrderBy(o=>o);
}
其對應TSQL的OrderBy敘述就可以正常被產生，TSQL敘述如下：
SELECT
[Distinct1].[Times] AS [Times]
FROM ( SELECT DISTINCT
[Extent1].[Times] AS [Times]
FROM [dbo].[conference_data] AS [Extent1]
) AS [Distinct1]
ORDER BY [Distinct1].[Times] ASC