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)
1: (from c in Customers
2: select new {
3: c.City,
4: c.ContactName
5: }).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)
1: (from c in Customers
2: select new {
3: c.City,
4: c.ContactName
5: }).Skip(5).Take(5)
Generated SQL
1: DECLARE @p0 Int = 5
2: DECLARE @p1 Int = 5
3:
4: SELECT [t1].[City], [t1].[ContactName]
5: FROM (
6: SELECT ROW_NUMBER() OVER (ORDER BY [t0].[City], [t0].[ContactName]) AS [ROW_NUMBER],
7: [t0].[City], [t0].[ContactName]
8: FROM [Customers] AS [t0]
9: ) AS [t1]
10: WHERE [t1].[ROW_NUMBER] BETWEEN @p0 + 1 AND @p0 + @p1
11: 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。
1: (from c in Customers
2: orderby c.ContactName
3: select new
4: {c.City, c.ContactName }).Skip(0).Take(5)
Generated SQL
1: DECLARE @p0 Int = 0
2: DECLARE @p1 Int = 5
3:
4: SELECT [t1].[City], [t1].[ContactName]
5: FROM (
6: SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ContactName]) AS [ROW_NUMBER], [t0].[City], [t0].[ContactName]
7: FROM [Customers] AS [t0]
8: ) AS [t1]
9: WHERE [t1].[ROW_NUMBER] BETWEEN @p0 + 1 AND @p0 + @p1
10: 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
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