如何执行 Linq to Entites 左外连接

发布于 2024-09-06 18:45:16 字数 764 浏览 8 评论 0原文

我已经阅读了大量博客文章，但尚未找到如何在两个表之间执行 LEFT OUTER JOIN 的清晰且简单的示例。关于连接的维基百科文章 Join (SQL) 提供了这个简单的模型：

CREATE TABLE `employee` (
`LastName` varchar(25),
`DepartmentID` int(4),
UNIQUE KEY `LastName` (`LastName`)
);

CREATE TABLE `department` (
`DepartmentID` int(4),
`DepartmentName` varchar(25),
UNIQUE KEY `DepartmentID` (`DepartmentID`)
);

假设我们有一个 EmployeeSet 作为员工容器 ObjectSet; EmployeeSet 和 DepartmentSet ObjectSet部门集。您将如何使用 Linq 执行以下查询？

SELECT LastName, DepartmentName
FROM employee e 
  LEFT JOIN department d 
    ON e.DepartmentID = d.DepartmentID

原文

I have read plenty of blog posts and have yet to find a clear and simple example of how to perform a LEFT OUTER JOIN between two tables. The Wikipedia article on joins Join (SQL) provides this simple model:

CREATE TABLE `employee` (
`LastName` varchar(25),
`DepartmentID` int(4),
UNIQUE KEY `LastName` (`LastName`)
);

CREATE TABLE `department` (
`DepartmentID` int(4),
`DepartmentName` varchar(25),
UNIQUE KEY `DepartmentID` (`DepartmentID`)
);

Assume we had a EmployeeSet as an employee container ObjectSet<Employee> EmployeeSet and a DepartmentSet ObjectSet<Department> DepartmentSet. How would you perform the following query using Linq?

SELECT LastName, DepartmentName
FROM employee e 
  LEFT JOIN department d 
    ON e.DepartmentID = d.DepartmentID

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仙女山的月亮 2024-09-13 18:45:16

我会写这个，它比 join 和做同样的事情：

var q = from e in db.EmployeeSet
        select new 
        {
            LastName = e.LastName,
            DepartmentName = e.Department.DepartmentName
        };

I would write this, which is far simpler than join and does exactly the same thing:

var q = from e in db.EmployeeSet
        select new 
        {
            LastName = e.LastName,
            DepartmentName = e.Department.DepartmentName
        };

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杀お生予夺 2024-09-13 18:45:16

您需要使用 DefaultIfEmpty 方法：

var query =
    from e in db.EmployeeSet
    join d in db.DepartmentSet on e.DepartmentID equals d.DepartmentID into temp
    from d in temp.DefaultIfEmpty()
    select new { Employee = e, Department = d };

You need to use the DefaultIfEmpty method :

var query =
    from e in db.EmployeeSet
    join d in db.DepartmentSet on e.DepartmentID equals d.DepartmentID into temp
    from d in temp.DefaultIfEmpty()
    select new { Employee = e, Department = d };

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