Linq语句实现(INNER JOIN、LEFT OUTER JOIN、RIGHT OUTER JOIN、FULL OUTER JOIN、CROSS JOIN)

首先建立三张表如下：

分别加入数据如下：

内连接 INNER JOIN

SELECT [t0].[Id], [t0].[Name], [t1].[GroupName], [t2].[Salary] AS [Content]
FROM [User] AS [t0]
INNER JOIN [Group] AS [t1] ON [t0].[GroupId] = ([t1].[Id])
INNER JOIN [Salary] AS [t2] ON ([t0].[Id]) = [t2].[UserId]

from u in Users
join g in Groups on u.GroupId  equals g.Id
join s in Salaries on u.Id equals s.UserId
select new
{
u.Id,
u.Name,
g.GroupName,
s.Content
}

Users
.Join (
Groups,
u => u.GroupId,
g => (Int32?)(g.Id),
(u, g) =>
new
{
u = u,
g = g
}
)
.Join (
Salaries,
temp0 => (Int32?)(temp0.u.Id),
s => s.UserId,
(temp0, s) =>
new
{
Id = temp0.u.Id,
Name = temp0.u.Name,
GroupName = temp0.g.GroupName,
Content = s.Content
}
)

左连接 Left Join

-- Region Parameters
DECLARE @p0 Decimal(1,0) = 0
-- EndRegion
SELECT [t0].[Id], [t0].[Name],
(CASE
WHEN [t2].[test] IS NULL THEN @p0
ELSE CONVERT(Decimal(29),[t2].[Salary])
END) AS [Content]
FROM [User] AS [t0]
LEFT OUTER JOIN (
SELECT 1 AS [test], [t1].[Salary], [t1].[UserId]
FROM [Salary] AS [t1]
) AS [t2] ON ([t0].[Id]) = [t2].[UserId]

from u in Users
join s in Salaries on u.Id equals s.UserId into NewSalaries
from n in NewSalaries.DefaultIfEmpty()
select new
{
u.Id,
u.Name,
Content = n==null?0:n.Content
}

Users
.GroupJoin (
Salaries,
u => (Int32?)(u.Id),
s => s.UserId,
(u, NewSalaries) =>
new
{
u = u,
NewSalaries = NewSalaries
}
)
.SelectMany (
temp0 => temp0.NewSalaries.DefaultIfEmpty (),
(temp0, n) =>
new
{
Id = temp0.u.Id,
Name = temp0.u.Name,
Content = (n == null) ? (Decimal?)0 : n.Content
}
)

右连接 RIGHT JOIN

混合使用

SELECT [t0].[Id], [t0].[Name], [t1].[GroupName],
(CASE
WHEN [t3].[test] IS NULL THEN @p0
ELSE CONVERT(Decimal(29),[t3].[Salary])
END) AS [Content]
FROM [User] AS [t0]
INNER JOIN [Group] AS [t1] ON [t0].[GroupId] = ([t1].[Id])
LEFT OUTER JOIN (
SELECT 1 AS [test], [t2].[Salary], [t2].[UserId]
FROM [Salary] AS [t2]
) AS [t3] ON ([t0].[Id]) = [t3].[UserId]

from u in Users
join g in Groups on u.GroupId  equals g.Id
join s in Salaries on u.Id equals s.UserId into NewSalaries
from n in NewSalaries.DefaultIfEmpty()
select new
{
u.Id,
u.Name,
g.GroupName,
Content = n==null?0:n.Content
}

Users
.Join (
Groups,
u => u.GroupId,
g => (Int32?)(g.Id),
(u, g) =>
new
{
u = u,
g = g
}
)
.GroupJoin (
Salaries,
temp0 => (Int32?)(temp0.u.Id),
s => s.UserId,
(temp0, NewSalaries) =>
new
{
<>h__TransparentIdentifier0 = temp0,
NewSalaries = NewSalaries
}
)
.SelectMany (
temp1 => temp1.NewSalaries.DefaultIfEmpty (),
(temp1, n) =>
new
{
Id = temp1.<>h__TransparentIdentifier0.u.Id,
Name = temp1.<>h__TransparentIdentifier0.u.Name,
GroupName = temp1.<>h__TransparentIdentifier0.g.GroupName,
Content = (n == null) ? (Decimal?)0 : n.Content
}
)