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我正在尝试在 LINQ to SQL 中编写以下查询。该表包含用户安排的会话列表，查询计算每个用户的连续会话之间的平均时间量。它使用左连接，以便只有一个会话的用户具有NULL值。

SELECT t1.workerId, AVG(DATEDIFF(s, t1.endTime, t2.startTime)) 
FROM e_userLongSessions t1 
LEFT JOIN e_userLongSessions t2 
ON t1.workerId = t2.workerId AND t1.sessionNum = t2.sessionNum - 1
GROUP BY t1.workerId
ORDER BY t1.workerId

基于问题LINQ to SQL Left Outer Join和How to do joins in LINQ on multiple fields in single join，我得到了以下查询：

from s1 in gzClasses.e_userLongSessions
join s2 in gzClasses.e_userLongSessions
    on new {w = s1.workerId, n = s1.sessionNum} equals new {w = s2.workerId, n = s2.sessionNum - 1}
    into joined
from s2 in joined.DefaultIfEmpty(null)
group new {s1, s2} by s1.workerId into g                   
select g.Average(e => e.s2 == null ? (double?) null : (e.s2.startTime - e.s1.endTime).TotalSeconds);

我收到一条Unsupported overload used for query operator 'DefaultIfEmpty'消息。有什么建议么？

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您的 LINQ 查询的结构与 T-SQL 查询不同。具体来说，您只包括s在分组中。事实上，s以一种误导的方式命名。应该是s2。包括两者：

from s1 in gzClasses.e_userLongSessions                    
join s2 in gzClasses.e_userLongSessions
    on new {w = s1.workerId, n = s1.sessionNum} 
equals new {w = s2.workerId, n = s2.sessionNum - 1}
    into joined                    
from s2Null in joined.DefaultIfEmpty()                    
group new {s1, s2Null} by s1.workerId into g                    
orderby g.Key // workerId
select g.Average(e => (e.s2Null.startTime - e.s1.endTime).TotalSeconds);

现在，您可以汇总两个表中的数据。我认为您的两个查询都没有考虑到s2.startTime可以为空的情况。但这是一个错误，不是问题的重点。

于 2014-03-21T22:32:24.610 回答