在实体框架中选择前5名

是的

[Person]
PersonID, EmailAddress, FirstName, LastName


[OnlineAccount]
OnlineAccountID, PersonID, Nickname

每个人允许拥有0-* OnlineAccount。

在使用 C # 的实体框架中，如何选择拥有最多帐户的前5个人？

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最佳答案

Try this:

var items = context.PersonSet.OrderByDescending(u => u.OnlineAccounts.Count).Take(5);

This returns IQueryable<Person>. It doesn't return results yet, because it implements deferred execution. It will be translated to SQL and executed when needed:

var metarializedItems = items.ToList(); // ToList forces execution

or 

foreach(var item in items) // foreach forces execution

Example above will translate to SQL similar to this one:

SELECT TOP 5 p.PersonID, p.EmailAddress, p.FirstName, p.LastName
FROM Person p
ORDER BY (SELECT COUNT(*) FROM OnlineAccount oa WHERE p.PersonID = oa.PersonID) DESC

It won't be this exact SQL. Different EF version may produce different SQLs, but I wrote it to ilustrate how it works. Take(5) is translated to TOP 5. OrderByDescending(u => u.OnlineAccounts.Count) is translated to ORDER BY (SELECT COUNT(*) FROM OnlineAccount oa WHERE p.PersonID = oa.PersonID) DESC. This is power of Entity Framework. It translates .NET expressions to SQL.


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