c# - 具有值或 null 的 Entity Framework LINQ

Question

我有以下 EF linq 查询来获取所有雇员或特定员工，以防 empId 有值(value)。

但是 EF 没有生成预期的查询，它总是忽略 OR 条件

from employee 
where employee.DepartmentId == depId && ((employee.Id == empId) || (employee.Id == null))
.ToList()

预期查询

SELECT * FROM Employee
WHERE DepartmentId = @DepId AND (Id=@empId OR Id IS NULL)

传递值时由 EF 生成查询

SELECT * FROM Employee
WHERE DepartmentId = @DepId AND (Id=@empId)

当值为 null 时由 EF 生成查询

SELECT * FROM Employee
WHERE DepartmentId = @DepId AND (Id IS NULL)

Note: No Employee record has Id value NULL and it returns 0 instead of all employees

如何为此 Id=@empId OR Id IS NULL 编写 linq？

Answer 1

你说这是你预期的查询:

SELECT * FROM Employee
WHERE DepartmentId = @DepId AND (Id=@empId OR Id IS NULL)

但我很确定它不是，因为 Id 永远不会为 null(您在 Note 中也说过)，因为它是那张 table 你实际上是指这个查询:

SELECT * FROM Employee
WHERE DepartmentId = @DepId AND (Id=@empId OR @empId IS NULL)

因此如果参数为NULL，您希望绕过检查并返回所有记录。请注意，这不是性能方面的最佳方式。当您不想按 Id 过滤时，您应该使用不带过滤器的查询。恐怕这不会产生最有效的查询计划。我会用这个:

IQueryable<Employee> allDepartmentEmployees = 
    from employee in ...
    where employee.DepartmentId == depId;
    select employee;

if(empId.HasValue) // presuming it's a nullable type
{
    allDepartmentEmployees = allDepartmentEmployees.Where(e => e.Id == empId.Value);     
}

List<Employee> employeeList = allDepartmentEmployees.ToList();

所以只有在给定参数的情况下才进行过滤。那么这只会返回一条记录。