Question

我的Distinct查询工作正常，但是我需要在另一个方法中放入WHERE子句。

这是我检索不同经理的工作查询

public static IEnumerable<DTO.Employee> GetDistinctMgrs()
{
    IEnumerable<DTO.Employee> managers = EmpCache.EmployeeList.GroupBy(x => x.MgrID).Select(x => x.FirstOrDefault());
    return managers;
}

在这里，我试图从哪里选拔不同的经理人名单中的雇员经理人。但是有了WHERE子句，我得到“枚举没有结果”

public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
    IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
            .Where(s => s.EmpID == EMPID)
            .GroupBy(x => x.MgrID)
    .Select(x => x.FirstOrDefault());

     return manager; //<- "Enumeration yielded no results"
}

Answer 1

您正在做的是向数据库发送请求，当请求完成时，您进行groupby等操作。

因此，当数据库不返回任何内容时，您会得到异常

这是我要怎么做

public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
 {
  IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
            .Where(s => s.EmpID == EMPID)
             .GroupBy(x => x.MgrID);
           if(manager?.Count()>0)
            return manager.Select(x => x.First());

       return null;
    }

Answer 2

我会做

 public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
 {
               IEnumerable<DTO.Employee> manager = 
               from mng in EmpCache.EmployeeList
                where mng.EmpID.ToLower() == EMPID.ToLower()
                orderby mng.MgrID
                select mng;
               return manager;
    }

或

public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
    IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
            .Where(s => s.EmpID.ToLower() == EMPID.ToLower())
            .GroupBy(x => x.MgrID)
    .Select(x => x.FirstOrDefault()).ToList();

     return manager; //<- "Enumeration yielded no results"
}

基于您的新问题

 public static  string GetDefaultMgr(string EMPID)
    {
       string managerID = EmpCache.EmployeeList
                .Where(s => s.EmpID.ToLower() == EMPID.ToLower())
                .GroupBy(x => x.MgrID)
        .Select(x => x.MgrID).FirstOrDefault().ToString();

         return managerID; //<- "Enumeration yielded no results"
    }

具有Distinct和WHERE子句的LINQ查询

2 个答案: