一、本将主要介绍内容
从linq，sql，lambda三个角度比较来学习
select、orderby、分页、group by、distinct、子查询、in的用法
1.select
查询用户和它们的自我介绍
Linq to sql
from a in Blog_UserInfoselect new{ 真实名字=a.RealName, 自我介绍=a.Introduce} sql
SELECT [t0].[RealName] AS [真实名字], [t0].[Introduce] AS [自我介绍]FROM [Blog_UserInfo] AS [t0]
Lambda
Blog_UserInfo .Select ( a => new { 真实名字 = a.RealName, 自我介绍 = a.Introduce } ) 2.orderby
查询名字里带friend的用户，并排序
Linq to sql
from a in Blog_Userswhere a.NickName.Contains("Friend")orderby a.UserId ascending,a.CreateTime descendingselect a--或者from a in Blog_Userswhere a.NickName.Contains("Friend")orderby a.UserId,a.CreateTime select a sql
-- Region ParametersDECLARE @p0 NVarChar(1000) = '%Friend%'-- EndRegionSELECT [t0].[UserId], [t0].[NickName], [t0].[CreateTime]FROM [Blog_User] AS [t0]WHERE [t0].[NickName] LIKE @p0ORDER BY [t0].[UserId], [t0].[CreateTime] DESC
Lambda
Blog_Users .Where (a => a.NickName.Contains ("Friend")) .OrderBy (a => a.UserId) .ThenByDescending (a => a.CreateTime) 3.分页
按照每页2条 ，查询第2页的留言表的信息
Linq to sql
(from a in Blog_LeaveMsgs select a).Skip(2).Take(2)
sql
-- Region ParametersDECLARE @p0 Int = 2DECLARE @p1 Int = 2-- EndRegionSELECT [t1].[ID], [t1].[ReceiverId], [t1].[LeaverId], [t1].[CreateTime], [t1].[Content]FROM ( SELECT ROW_NUMBER() OVER (ORDER BY [t0].[ID], [t0].[ReceiverId], [t0].[LeaverId], [t0].[CreateTime], [t0].[Content]) AS [ROW_NUMBER], [t0].[ID], [t0].[ReceiverId], [t0].[LeaverId], [t0].[CreateTime], [t0].[Content] FROM [Blog_LeaveMsg] AS [t0] ) AS [t1]WHERE [t1].[ROW_NUMBER] BETWEEN @p0 + 1 AND @p0 + @p1ORDER BY [t1].[ROW_NUMBER]
Lambda
Blog_LeaveMsgs .Select (a => a) .Skip (2) .Take (2)
4.1分组1（group by字段）
根据用户来分组，查询留言数大于等于3条的用户ID和相应留言数量
Linq to sql
from a in Blog_LeaveMsgs group a by a.LeaverId into bwhere b.Count() >=3select new{ 朋友ID = b.Key, 留言数 = b.Count()} sql
-- Region ParametersDECLARE @p0 Int = 3-- EndRegionSELECT [t1].[LeaverId] AS [朋友ID], [t1].[value2] AS [留言数]FROM ( SELECT COUNT(*) AS [value], COUNT(*) AS [value2], [t0].[LeaverId] FROM [Blog_LeaveMsg] AS [t0] GROUP BY [t0].[LeaverId] ) AS [t1]WHERE [t1].[value] >= @p0
4.2分组2（group by多个字段）
按照接收人和留言人进行分组，查看覆盖的接收人和留言人情况
Linq to sql
from a in Blog_LeaveMsgsgroup a by new{a.ReceiverId,a.LeaverId} into bselect new{ 接收人ID=b.Key.ReceiverId, 留言人ID=b.Key.LeaverId} sql
SELECT [t0].[ReceiverId] AS [接收人ID], [t0].[LeaverId] AS [留言人ID]FROM [Blog_LeaveMsg] AS [t0]GROUP BY [t0].[ReceiverId], [t0].[LeaverId]
Lambda
Blog_LeaveMsgs .GroupBy ( a => new { ReceiverId = a.ReceiverId, LeaverId = a.LeaverId } ) .Select ( b => new { 接收人ID = b.Key.ReceiverId, 留言人ID = b.Key.LeaverId } ) 5.distinct
查看留言表中的留言人人数
Linq to sql
(from a in Blog_LeaveMsgsselect a.LeaverId).Distinct()
sql
SELECT DISTINCT [t0].[LeaverId]FROM [Blog_LeaveMsg] AS [t0]
Lambda
Blog_LeaveMsgs .Select (a => a.LeaverId) .Distinct ()
6.子查询
查询留言数量超过4条的用户信息
Linq to sql
from a in Blog_Userswhere(from b in Blog_LeaveMsgs group b by b.LeaverId into b where b.Count()>=4select b.Key).Contains(a.UserId)select a
sql
-- Region ParametersDECLARE @p0 Int = 4-- EndRegionSELECT [t0].[UserId], [t0].[NickName], [t0].[CreateTime]FROM [Blog_User] AS [t0]WHERE EXISTS( SELECT NULL AS [EMPTY] FROM ( SELECT COUNT(*) AS [value], [t1].[LeaverId] FROM [Blog_LeaveMsg] AS [t1] GROUP BY [t1].[LeaverId] ) AS [t2] WHERE ([t2].[LeaverId] = ([t0].[UserId])) AND ([t2].[value] >= @p0) )
Lambda
Blog_Users .Where ( a => Blog_LeaveMsgs .GroupBy (b => b.LeaverId) .Where (b => (b.Count () >= 4)) .Select (b => b.Key) .Contains ((Int32?)(a.UserId)) )
7.in操作
查询制定用户昵称的用户
Linq to sql
from a in Blog_Userswhere new string[]{ "Kimisme","FriendLee"}.Contains(a.NickName)select a sql
-- Region ParametersDECLARE @p0 NVarChar(1000) = 'Kimisme'DECLARE @p1 NVarChar(1000) = 'FriendLee'-- EndRegionSELECT [t0].[UserId], [t0].[NickName], [t0].[CreateTime]FROM [Blog_User] AS [t0]WHERE [t0].[NickName] IN (@p0, @p1)
Lambda
Blog_Users .Where (a => new String[] { "Kimisme", "FriendLee" } .Contains (a.NickName))