EFCore某张表中获取某几个字段

EFCore某张表中获取某几个字段

1.背景

在前后端分离的应用场景中,某张统计表有20几个字段,但是前端可能只用到4个字段,这样就涉及到获取某个表中的部分字段值。本文介绍3种方法。

2.法一:linq

2.1 使用Select方法

List<Emplayee> emplayeeList = GetEmplayeeList();            //获取员工信息列表
int[] empIds = emplayeeList.Select(a => a.Id).ToArray();    //获取员工编号数组

2.2 使用ForEach方法

List<Emplayee> emplayeeList = GetEmplayeeList();  //获取员工信息列表
string empIds = "";
emplayeeList.ForEach(a => empIds += a.Id + ",");
empIds = empIds.TrimEnd(',');
Console.WriteLine(empIds);   //输出:1,2,3

2.3 其他参考代码

/// <summary>  
/// 员工信息类  
/// </summary>  
public class Emplayee
{
    /// <summary>
    /// 编号
    /// </summary>
    public int Id { get; set; }
 
    /// <summary>  
    /// 姓名  
    /// </summary>  
    public string Name { get; set; }


/// <summary>  
/// 获取员工信息列表  
/// </summary>  
/// <returns></returns>  
public static List<Emplayee> GetEmplayeeList()
{
    List<Emplayee> emplayeeList = new List<Emplayee>();
    Emplayee emplayee1 = new Emplayee() { Id = 1, Name = "张三" };
    Emplayee emplayee2 = new Emplayee() { Id = 2, Name = "李四" };
    Emplayee emplayee3 = new Emplayee() { Id = 3, Name = "王五" };
    emplayeeList.Add(emplayee1);
    emplayeeList.Add(emplayee2);
    emplayeeList.Add(emplayee3);
    return emplayeeList;
}

3.法二:IQueryble

3.1 参考例子一

我有个新闻表

id,title,body,createtime,author,click

使用ef4.1 仅仅读取 id,title,createtime 并显示在页面上。

 public static List<NewInfo> GetHotNews()
        {
            List<NewInfo> list;
            list = (from n in db.NewInfoes
                    where n.IsTop == 1
                    orderby n.PublishTime descending
                    select new
                    {
                        Title = n.Title,
                        NewID = n.NewID,
                        PublishTime = n.PublishTime
                    }).ToList();
            return list;
        }
改为
public static List<NewInfo> GetHotNews()
        {
            List<NewInfo> list;
            list = (from n in db.NewInfoes
                    where n.IsTop == 1
                    orderby n.PublishTime descending
                   
       .Select(n => new NewInfo
                    {
                        Title = n.Title,
                        NewID = n.NewID,
                        PublishTime = n.PublishTime
                    });
            return list;
        }

备注:只是IEnumerable是把所有数据拉到内存里筛选,IQueryable会形成sql直接在库里查询返回

3.2 参考例子二

4.法三:建立一个对象,然后配置一个automapper,然后将查询结果匹配过来

具体可以自行尝试

参考:
1.网友 Kawhi(1585955375), 奥特曼迪斯特洛夫斯基(479663032)
2.https://blog.csdn.net/lcnmdfx/article/details/8332401
3.https://blog.csdn.net/pan_junbiao/article/details/51757904

posted @ 2019-10-22 15:49  JerryMouseLi  阅读(4478)  评论(12编辑  收藏  举报