EFCore 获取两个表之间的差异

Question

我试图找出两个 table 之间的区别，我们称它们为 YestedayEmployees 和 TodaysEmployees。我目前正在获取所有 table，然后检查每个项目以查看员工状态是否更改或是否被删除（它们出现在昨天但不在今天的 table 中）。当记录数量较少时这很好，但随着记录的增加，我服务器上的网络和计算开销正成为一个问题。有没有办法在 EFCore 中作为 linq 查询来执行此操作？（甚至两个，一个用于删除，一个用于更改）

Answer 1

请参考以下查询语句：

测试数据（您可以使用 EF 核心从数据库中获取 table 值，有关将 EF 核心与 asp.net MVC 结合使用的更多详细信息，检查 this link）：

        List<Employee> todayEmployees = new List<Employee>()
        {
            new Employee(){ EmpID=1001, EmpName="David", Status="OT" },
            new Employee(){ EmpID=1002, EmpName="Tom", Status="Off-line" },
            new Employee(){ EmpID=1003, EmpName="Jason", Status="OT" },
            new Employee(){ EmpID = 1004, EmpName="Dick", Status="Off-line" },
            new Employee(){ EmpID = 1005, EmpName="Cece", Status="OT" },
            new Employee(){ EmpID = 1006, EmpName="Dillion", Status="OT" },
            new Employee(){ EmpID = 1007, EmpName="Jeffery", Status="Off-Line" }
        };

        List<Employee> yesterdayEmployees = new List<Employee>()
        {
            new Employee(){ EmpID=1001, EmpName="David", Status="OT" },
            new Employee(){ EmpID=1002, EmpName="Tom", Status="OT" },
            new Employee(){ EmpID=1003, EmpName="Jason", Status="OT"},
            new Employee(){ EmpID = 1004, EmpName="Dick", Status="OT" },
            new Employee(){ EmpID = 1005, EmpName="Cece", Status="Off-Line" }
        };

要获取状态发生变化的Employee，我们可以使用Join子句和where子句来比较员工状态：

        // get the employees which changes status
        var result = (from t in todayEmployees
                      join y in yesterdayEmployees
                      on t.EmpID equals y.EmpID
                      where (t.Status != y.Status)
                      select t).ToList();

输出：

        //get the employees status change information
        var query3 = (from t in todayEmployees
                      join y in yesterdayEmployees
                      on t.EmpID equals y.EmpID
                      where (t.Status != y.Status)
                      select new EmployeeViewModel()
                      {
                          EmpID = t.EmpID,
                          EmpName = t.EmpName,
                          StatusChangeLog = "change status from " + t.Status + " to " + y.Status
                      }).ToList();

输出：

要获取在TodayEmployeesTable中但在YesterdayEmployeeTable中不存在的Employees，我们可以使用contains method来判断一个序列是否包含指定的元素。

        //get the employees, which in TodayEmployees Table, but not exist in the YesterdayEmployee
        var query4 = (from t in todayEmployees 
                      where !(from y in yesterdayEmployees select y.EmpID).Contains(t.EmpID)
                      select t).ToList();

        var query5 = todayEmployees.Where(c => !yesterdayEmployees.Select(y => y.EmpID).Contains(c.EmpID)).Select(t => t).ToList();

输出：

EFCore 获取两个表之间的差异

EFCore Getting Differences between two tables

linq

entity-framework-core

.net-core

asp.net-core

ef-core-3.1