c# - 复杂的 Linq 查询

Question

我在数据库中有一个包含 2 个字段的表：索引（int），电子邮件（varchar（100））

我需要执行以下操作：

1. 按域名对所有电子邮件进行分组（所有电子邮件都已小写）。
2. 从所有组中选择所有电子邮件，其中域的电子邮件总和不超过步骤 1 之前电子邮件总数的 20%。

代码示例：

    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();
        }
    }

有谁知道更好的制作方法吗？

Answer 1

如果不至少两次访问数据库，我想不出一种方法，一次用于分组，一次用于总体计数，您可以尝试类似的方法

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

假设您要获取每个组中按升序排列的最后一项：

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

或这个：

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

Answer 3

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);