C# LINQ to SQL：GroupBy() 和 Max() 以获取具有最新日期的对象
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LINQ to SQL: GroupBy() and Max() to get the object with latest date
提问by p.campbell
Consider a SQL Server table that's used to store events for auditing.
考虑一个用于存储审计事件的 SQL Server 表。
The need is to get only that latest entryfor each CustID. We want to get the entire object/row. I am assuming that a GroupBy() will be needed in the query. Here's the query so far:
需要只获取每个 CustID 的最新条目。我们想要获取整个对象/行。我假设查询中将需要 GroupBy()。这是到目前为止的查询：
var custsLastAccess = db.CustAccesses
.Where(c.AccessReason.Length>0)
.GroupBy(c => c.CustID)
// .Select()
.ToList();
// (?) where to put the c.Max(cu=>cu.AccessDate)
Question:How can I create the query to select the latest(the maximum AccessDate) record/object for each CustID?
问题：如何创建查询来AccessDate为每个选择最新（最大）记录/对象CustID？
采纳答案by Marc Gravell
I'm wondering if something like:
我想知道是否有类似的东西：
var custsLastAccess = db.CustAccesses
.Where(c.AccessReason.Length>0)
.GroupBy(c => c.CustID)
.Select(grp => new {
grp.Key,
LastAccess = grp
.OrderByDescending(x => x.AccessDate)
.Select(x => x.AccessDate)
.FirstOrDefault()
}).ToList();
you could also try OrderBy()and Last()
你也可以尝试OrderBy()和Last()
回答by CodeGrue
Using LINQ syntax, which I think looks cleaner:
使用 LINQ 语法，我认为它看起来更简洁：
var custsLastAccess = from c in db.CustAccesses
group c by c.CustID into grp
select grp.OrderByDescending(c => c.AccessDate).FirstOrDefault();
回答by Gary
Here: this uses max rather than OrderByDesc, so should be more efficient.
这里：这使用 max 而不是 OrderByDesc，所以应该更有效。
var subquery = from c in CustAccesses
group c by c.CustID into g
select new
{
CustID = g.Key,
AccessDate = g.Max(a => a.AccessDate)
};
var query = from c in CustAccesses
join s in subquery
on c.CustID equals s.CustID
where c.AccessDate == s.AccessDate
&& !string.IsNullOrEmpty(c.AccessReason)
select c;
