使用INNER JOIN時,結果集中只包含相符的元素。不匹配的元素將從結果集中排除。
使用LEFT OUTER JOIN時,所有匹配元素左側集合中的所有不匹配元素都將包含在結果集中.
讓我們透過一個範例來了解左外連接的實作。考慮以下 Department 和 Employee 類別。請注意,員工 Mary 沒有分配部門。內連接不會將她的記錄包含在結果集中,而左外連接則會將她的記錄包含在內。
static class Program{ static void Main(string[] args){ var result = Employee.GetAllEmployees() .GroupJoin(Department.GetAllDepartments(), e => e.DepartmentID, d => d.ID, (emp, depts) => new { emp, depts }) .SelectMany(z => z.depts.DefaultIfEmpty(), (a, b) => new{ EmployeeName = a.emp.Name, DepartmentName = b == null ? "No Department" : b.Name }); foreach (var v in result){ Console.WriteLine(" " + v.EmployeeName + "\t" + v.DepartmentName); } } } public class Department{ public int ID { get; set; } public string Name { get; set; } public static List<Department> GetAllDepartments(){ return new List<Department>(){ new Department { ID = 1, Name = "IT"}, new Department { ID = 2, Name = "HR"}, }; } } public class Employee{ public int ID { get; set; } public string Name { get; set; } public int DepartmentID { get; set; } public static List<Employee> GetAllEmployees(){ return new List<Employee>(){ new Employee { ID = 1, Name = "Mark", DepartmentID = 1 }, new Employee { ID = 2, Name = "Steve", DepartmentID = 2 }, new Employee { ID = 3, Name = "Ben", DepartmentID = 1 }, new Employee { ID = 4, Name = "Philip", DepartmentID = 1 }, new Employee { ID = 5, Name = "Mary" } }; } }
以上是如何在 C# 中使用 linq 擴充方法執行左外連接?的詳細內容。更多資訊請關注PHP中文網其他相關文章!