标签：ram join oid 建议 nts 官方 graph exec 投影
Entity Framework Core中的查询与EF 6.x中的查询相同，但具有更优化的SQL查询，并且能够将C＃/ VB.NET函数包含在LINQ到实体查询中。
Entity Framework Core 允许你在模型中使用导航属性来加载相关实体。 有三种常见的 O/RM 模式可用于加载关联数据:
访问LINQ-to-Entities一章，以了解有关Entity Framework中查询基础的更多信息。
在这里，您将学习Entity Framework Core中引入的查询新功能。
EF Core在LINQ-to-Entities中具有新功能，我们可以在查询中包含C＃或VB.NET函数。这在EF 6中是不可能的。
private static void Main(string[] args)
{
var context = new SchoolContext();
var studentsWithSameName = context.Students
.Where(s => s.FirstName == GetName())
.ToList();
}
public static string GetName() {
return "Bill";
}在上面的L2E查询中，我们在Where子句中包含了GetName() C＃函数。这将在数据库中执行以下查询：
exec sp_executesql N'SELECT [s].[StudentId], [s].[DoB], [s].[FirstName],
[s].[GradeId], [s].[LastName], [s].[MiddleName]
FROM [Students] AS [s]
WHERE [s].[FirstName] = @__GetName_0',N'@__GetName_0 nvarchar(4000)',
@__GetName_0=N'Bill'
GoEntity Framework Core支持使用Include()扩展方法和投影查询来快速加载与EF 6相同的相关实体。除此之外，它还提供了ThenInclude()扩展方法来加载多个级别的相关实体。 (EF 6不支持ThenInclude()方法。)
与EF 6不同，我们可以在Include（）方法中将lambda表达式指定为参数，以指定导航属性，如下所示。
var context = new SchoolContext();
var studentWithGrade = context.Students
.Where(s => s.FirstName == "Bill")
.Include(s => s.Grade)
.FirstOrDefault();在上面的示例中，.Include(s => s.Grade)传递lambda表达式s => s.Grade，以指定引用属性，该属性将在单个SQL查询中与来自数据库的Student实体数据一起加载。上面的查询在数据库中执行以下SQL查询。
SELECT TOP(1) [s].[StudentId], [s].[DoB], [s].[FirstName], [s].[GradeId],[s].[LastName],
[s].[MiddleName], [s.Grade].[GradeId], [s.Grade].[GradeName], [s.Grade].[Section]
FROM [Students] AS [s]
LEFT JOIN [Grades] AS [s.Grade] ON [s].[GradeId] = [s.Grade].[GradeId]
WHERE [s].[FirstName] = N'Bill'我们还可以在Include（）方法中将属性名称指定为字符串，与EF 6中相同。
var context = new SchoolContext();
var studentWithGrade = context.Students
.Where(s => s.FirstName == "Bill")
.Include("Grade")
.FirstOrDefault();不建议使用上面的示例，因为如果属性名称拼写错误或不存在，则会抛出运行时异常。始终对lambda表达式使用Include（）方法，以便可以在编译时检测到错误。
Include（）扩展方法也可以在FromSql（）方法之后使用，如下所示。
var context = new SchoolContext();
var studentWithGrade = context.Students
.FromSql("Select * from Students where FirstName ='Bill'")
.Include(s => s.Grade)
.FirstOrDefault(); 注意：DbSet.Find()方法之后不能使用Include()扩展方法。例如。在EF Core 2.0中无法使用context.Students.Find（1）.Include（）。在将来的版本中这可能是可能的。
多次使用Include（）方法来加载同一实体的多个导航属性。例如，以下代码加载与Student的Grade和StudentCourses相关的实体。
var context = new SchoolContext();
var studentWithGrade = context.Students.Where(s => s.FirstName == "Bill")
.Include(s => s.Grade)
.Include(s => s.StudentCourses)
.FirstOrDefault();上面的查询将在单个数据库往返中执行两个SQL查询。
SELECT TOP(1) [s].[StudentId], [s].[DoB], [s].[FirstName], [s].[GradeId], [s].[LastName],
[s].[MiddleName], [s.Grade].[GradeId], [s.Grade].[GradeName], [s.Grade].[Section]
FROM [Students] AS [s]
LEFT JOIN [Grades] AS [s.Grade] ON [s].[GradeId] = [s.Grade].[GradeId]
WHERE [s].[FirstName] = N'Bill'
ORDER BY [s].[StudentId]
Go
SELECT [s.StudentCourses].[StudentId], [s.StudentCourses].[CourseId]
FROM [StudentCourses] AS [s.StudentCourses]
INNER JOIN (
SELECT DISTINCT [t].*
FROM (
SELECT TOP(1) [s0].[StudentId]
FROM [Students] AS [s0]
LEFT JOIN [Grades] AS [s.Grade0] ON [s0].[GradeId] = [s.Grade0].[GradeId]
WHERE [s0].[FirstName] = N'Bill'
ORDER BY [s0].[StudentId]
) AS [t]
) AS [t0] ON [s.StudentCourses].[StudentId] = [t0].[StudentId]
ORDER BY [t0].[StudentId]
GoEF Core引入了新的ThenInclude（）扩展方法，以加载多个级别的相关实体。考虑以下示例：
var context = new SchoolContext();
var student = context.Students.Where(s => s.FirstName == "Bill")
.Include(s => s.Grade)
.ThenInclude(g => g.Teachers)
.FirstOrDefault();在上面的示例中，.Include(s => s.Grade)将加载Student实体的Grade导航属性。 .ThenInclude(g => g.Teachers)将加载Grade实体的Teacher集合属性。必须在Include方法之后调用ThenInclude方法。上面的代码将在数据库中执行以下SQL查询。
SELECT TOP(1) [s].[StudentId], [s].[DoB], [s].[FirstName], [s].[GradeId], [s].[LastName],
[s].[MiddleName], [s.Grade].[GradeId], [s.Grade].[GradeName], [s.Grade].[Section]
FROM [Students] AS [s]
LEFT JOIN [Grades] AS [s.Grade] ON [s].[GradeId] = [s.Grade].[GradeId]
WHERE [s].[FirstName] = N'Bill'
ORDER BY [s.Grade].[GradeId]
Go
SELECT [s.Grade.Teachers].[TeacherId], [s.Grade.Teachers].[GradeId], [s.Grade.Teachers].[Name]
FROM [Teachers] AS [s.Grade.Teachers]
INNER JOIN (
SELECT DISTINCT [t].*
FROM (
SELECT TOP(1) [s.Grade0].[GradeId]
FROM [Students] AS [s0]
LEFT JOIN [Grades] AS [s.Grade0] ON [s0].[GradeId] = [s.Grade0].[GradeId]
WHERE [s0].[FirstName] = N'Bill'
ORDER BY [s.Grade0].[GradeId]
) AS [t]
) AS [t0] ON [s.Grade.Teachers].[GradeId] = [t0].[GradeId]
ORDER BY [t0].[GradeId]
go我们还可以通过使用投影查询而不是Include()或ThenInclude()方法来加载多个相关实体。以下示例演示了用于加载“学生”，“年级”和“教师”实体的投影查询。
var context = new SchoolContext();
var stud = context.Students.Where(s => s.FirstName == "Bill")
.Select(s => new
{
Student = s,
Grade = s.Grade,
GradeTeachers = s.Grade.Teachers
})
.FirstOrDefault();在上面的示例中，.Select扩展方法用于在结果中包括Student, Grade and Teacher实体。这将执行与上述ThenInclude（）方法相同的SQL查询。
关于延迟加载(Lazy Loading)和显式加载( Explicit Loading). EF Core 3.1 已经支持，原文为2.0版本已过时，可以参考官方文档：
参考: https://docs.microsoft.com/en-us/ef/core/querying/related-data
Entity Framework Core系列教程-6-查询数据
标签：ram join oid 建议 nts 官方 graph exec 投影
原文地址：https://www.cnblogs.com/AlexanderZhao/p/12289193.html
