复杂LINQ查询

Question

我有2个字段的数据库表：指数（INT），电子邮件（VARCHAR（100））

我需要做到以下几点：

1. 集团所有电子邮件域名（所有电子邮件已经小写）。
2. 选择从所有组中的所有电子邮件，其中电子邮件域不超过总邮件的20％步骤之前1.

代码示例的总和：

DataContext db = new DataContext(); 

    //Domains to group by 
    List<string> domains = new List<string>() { "gmail.com", "yahoo.com", "hotmail.com" }; 

    Dictionary<string, List<string>> emailGroups = new Dictionary<string, List<string>>(); 

    //Init dictionary 
    foreach (string thisDomain in domains) 
    { 
     emailGroups.Add(thisDomain, new List<string>()); 
    } 

    //Get distinct emails 
    var emails = db.Clients.Select(x => x.Email).Distinct(); 

    //Total emails 
    int totalEmails = emails.Count(); 

    //One percent of total emails 
    int onePercent = totalEmails/100; 

    //Run on each email 
    foreach (var thisEmail in emails) 
    { 
     //Run on each domain 
     foreach (string thisDomain in emailGroups.Keys) 
     { 
      //If email from this domain 
      if (thisEmail.Contains(thisDomain)) 
      { 
       //Add to dictionary 
       emailGroups[thisDomain].Add(thisEmail); 
      } 
     } 
    } 

    //Will store the final result 
    List<string> finalEmails = new List<string>(); 

    //Run on each domain 
    foreach (string thisDomain in emailGroups.Keys) 
    { 
     //Get percent of emails in group 
     int thisDomainPercents = emailGroups[thisDomain].Count/onePercent; 

     //More than 20% 
     if (thisDomainPercents > 20) 
     { 
      //Take only 20% and join to the final result 
      finalEmails = finalEmails.Union(emailGroups[thisDomain].Take(20 * onePercent)).ToList(); 
     } 
     else 
     { 
      //Join all to the final result 
      finalEmails = finalEmails.Union(emailGroups[thisDomain]).ToList(); 
     } 
    } 

有谁知道一个更好的办法做了？

Answer 1

我想不出这样做没有击中DB至少两次，一次用于分组和一个用于整体计数的方式，你可以尝试像

var query = from u in db.Users 
      group u by u.Email.Split('@')[1] into g 
      select new 
      { 
       Domain = g.Key, 
       Users = g.ToList() 
      }; 

query = query.Where(x => x.Users.Count <= (db.Users.Count() * 0.2)); 

Answer 2

var maxCount = db.Users.Count() * 0.2; 
var query = (from u in db.Users 
     group u by u.Email.Split('@')[1] into g 
     select new 
     { 
      Domain = g.Key, 
      Users = g.Take(maxCount).ToList() 
     }) 
     .SelectMany(x => x.Users); 

Answer 3

假设你想获得各组按升序最后一个项目：

int m = (int) (input.Count() * 0.2); 
var result = input.GroupBy(x=>x.email.Split('@')[1], 
          (key,g)=>g.OrderByDescending(x=>x.index).Take(m) 
            .OrderBy(x=>x.index)) 
        .SelectMany(g=>g);//If you want to get the last result without grouping 

或者这样：

var result = input.GroupBy(x=>x.email.Split('@')[1], 
          (key,g)=>g.OrderBy(x=>x.index) 
            .Skip(g.Count()-m)) 
        .SelectMany(g=>g);//If you want to get the last result without grouping