Linq to Sql 与Linq to Entities 生成的SQL Script与分页实现

      Linq to sql 与 Linq to entities 生成的SQL Script与分页实现，我们选用Northwind做演示的Database.

First 5 Records (Page 1)

(from c in Customers
        select new {
            c.City,
            c.ContactName
        }).Skip(0).Take(5)

Generated SQL

   1:  SELECT TOP (5) [t0].[City], [t0].[ContactName]

   2:  FROM [Customers] AS [t0]

Result set

City            ContactName
--------------- ------------------------------
Berlin          Maria Anders
México D.F.     Ana Trujillo
México D.F.     Antonio Moreno
London          Thomas Hardy
Luleå           Christina Berglund

Next 5 Records (Page 2)

(from c in Customers
        select new {
            c.City,
            c.ContactName
        }).Skip(5).Take(5)

Generated SQL

DECLARE @p0 Int = 5
DECLARE @p1 Int = 5

SELECT [t1].[City], [t1].[ContactName]
FROM (
    SELECT ROW_NUMBER() OVER (ORDER BY [t0].[City], [t0].[ContactName]) AS [ROW_NUMBER],
    [t0].[City], [t0].[ContactName]
    FROM [Customers] AS [t0]
    ) AS [t1]
WHERE [t1].[ROW_NUMBER] BETWEEN @p0 + 1 AND @p0 + @p1
ORDER BY [t1].[ROW_NUMBER]

Result set

City            ContactName
--------------- ------------------------------
Barquisimeto    Carlos González
Bergamo         Giovanni Rovelli
Berlin          Maria Anders
Bern            Yang Wang
Boise           Jose Pavarotti

    由此可以，可以用skip,take方法来实现分页，但也许你注意到上面有一条记录出现了两次： Berlin    Maria Anders,这是因为我们没有排序的原因。

在Linq to entities中也有这个问题，并且强制要求你要排序，否则throw出这个异常：

The method 'Skip' is only supported for sorted input in LINQ to Entities.
The method 'OrderBy' must be called before the method 'Skip'.

据说这个问题已经在.NET 4.0 framework修正了。

(from c in Customers
orderby c.ContactName
select new
{c.City, c.ContactName }).Skip(0).Take(5)

 
Generated SQL

DECLARE @p0 Int = 0
DECLARE @p1 Int = 5

SELECT [t1].[City], [t1].[ContactName]
FROM (
    SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ContactName]) AS [ROW_NUMBER], [t0].[City], [t0].[ContactName]
    FROM [Customers] AS [t0]
    ) AS [t1]
WHERE [t1].[ROW_NUMBER] BETWEEN @p0 + 1 AND @p0 + @p1
ORDER BY [t1].[ROW_NUMBER]

可以看出也用ROW_NUMBER实现的。另外，如果你不写Ordery，Linq to sql 生成的sql script将会非常不易阅读。

最后调用的CSharp代码可以是:

            int totalcount = dboperator.Customers.Count();
            int pagesize = 10;
            int pages = (int)Math.Round((double)totalcount / pagesize);

            for (int pageindex = 1; i <= pages; i++)
            {
                var customers = dboperator.Customers.OrderBy(c => c.ContactName)
                                   .Skip((pageindex - 1) * pageSize)
                                   .Take(pageSize);

                //...
            }

希望这篇Post对您有帮助.
Author: Petter Liu    http://wintersun.cnblogs.com