Linq学习-基本查询

Posted 2020-06-11

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_UserInfo
select 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_Users
where a.NickName.Contains("Friend")
orderby a.UserId ascending,
a.CreateTime descending
select a
--或者
from a in Blog_Users
where a.NickName.Contains("Friend")
orderby a.UserId,a.CreateTime 
select a

sql

-- Region Parameters
DECLARE @p0 NVarChar(1000) = ‘%Friend%‘
-- EndRegion
SELECT [t0].[UserId], [t0].[NickName], [t0].[CreateTime]
FROM [Blog_User] AS [t0]
WHERE [t0].[NickName] LIKE @p0
ORDER 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 Parameters
DECLARE @p0 Int = 2
DECLARE @p1 Int = 2
-- EndRegion
SELECT [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 + @p1
ORDER 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 b
where b.Count() >=3
select new
{
    朋友ID = b.Key,
    留言数 = b.Count()
}

sql

-- Region Parameters
DECLARE @p0 Int = 3
-- EndRegion
SELECT [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_LeaveMsgs
group a by new{a.ReceiverId,a.LeaverId} into b
select 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_LeaveMsgs
select 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_Users
where
(from b in Blog_LeaveMsgs 
group b by b.LeaverId into b 
where b.Count()>=4
select b.Key).Contains(a.UserId)
select a

sql

-- Region Parameters
DECLARE @p0 Int = 4
-- EndRegion
SELECT [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_Users
where new string[]{"Kimisme","FriendLee"}
.Contains(a.NickName)
select a

sql

-- Region Parameters
DECLARE @p0 NVarChar(1000) = ‘Kimisme‘
DECLARE @p1 NVarChar(1000) = ‘FriendLee‘
-- EndRegion
SELECT [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))