C# LINQ 分组，每组物品数量相同

Question

I cannot figure out how to group items in a correct way like this. For instance we have a list of dates and teams like this:

Date	Teams	Value
1/1/2021	Team1	10
1/1/2021	Team2	10
1/1/2021	Team3	10
1/2/2021	Team1	20
1/2/2021	Team2	20
1/3/2021	Team3	30

I want it to be like this:

Date	Teams	Value
1/1/2021	Team1	10
1/1/2021	Team2	10
1/1/2021	Team3	10
1/2/2021	Team1	20
1/2/2021	Team2	20
1/2/2021	Team3	0
1/3/2021	Team1	0
1/3/2021	Team2	0
1/3/2021	Team3	30

Answer 1

In C#:

public class DataEntry
{
    public string Date;
    public string Teams;
    public int Value;
}

public static IEnumerable<DataEntry> GetData()
{
    var data = new List<DataEntry>()
    {
        new DataEntry() { Date = "1/1/2021", Teams = "Team1", Value = 10 },
        new DataEntry() { Date = "1/1/2021", Teams = "Team2", Value = 10 },
        new DataEntry() { Date = "1/1/2021", Teams = "Team3", Value = 10 },
        new DataEntry() { Date = "1/2/2021", Teams = "Team1", Value = 20 },
        new DataEntry() { Date = "1/2/2021", Teams = "Team2", Value = 20 },
        new DataEntry() { Date = "1/3/2021", Teams = "Team3", Value = 30 },
    };

    var dates = data.Select(d => d.Date).Distinct().OrderBy(d => d);
    var teams = data.Select(d => d.Teams).Distinct().OrderBy(d => d);

    foreach (var date in dates)
    {
        foreach (var team in teams)
        {
            yield return new DataEntry()
            {
                Date = date,
                Teams = team,
                Value = data.SingleOrDefault(d => d.Date == date && d.Teams == team)?.Value ?? 0
            };
        }
    }
}

public static void Main(string[] args)
{
    foreach (var d in GetData()) Console.WriteLine($"{d.Date} {d.Teams} {d.Value}");
}

Assuming you have it in a database, it may be easier to do it there

SELECT t.Date, d.Teams, IsNull(tt.Value, 0) as Value
FROM (SELECT DISTINCT Date FROM #Table) t
  CROSS JOIN (SELECT DISTINCT Teams FROM #Table) d
  LEFT JOIN #Table tt
    ON t.Date = tt.Date
    AND d.Teams = tt.Teams
ORDER BY t.Date, d.Teams

Answer 2

Assuming your data looks like this:

var data = new[]
{
    new { Date = "1/1/2021", Team = "Team1", Value = 10 },
    new { Date = "1/1/2021", Team = "Team2", Value = 10 },
    new { Date = "1/1/2021", Team = "Team3", Value = 10 },
    new { Date = "1/2/2021", Team = "Team1", Value = 20 },
    new { Date = "1/2/2021", Team = "Team2", Value = 20 },
    new { Date = "1/3/2021", Team = "Team3", Value = 30 }
};

Then this will produce the table you want:

var teams = data.Select(rec => rec.Team).Distinct().OrderBy(team => team).ToArray();
var groupings = data.GroupBy(rec => DateTime.Parse(rec.Date));
var table = groupings.OrderBy(grouping => grouping.Key).SelectMany(grouping =>
    {
        var dict = grouping.ToDictionary(rec => rec.Team, rec => rec.Value);
        return teams.Select(team =>
            {
                dict.TryGetValue(team, out int value);
                return new { Date = grouping.Key, Team = team, Value = value };
            });
    });

The first statement gives you all the teams in alphabetical order. The second statement groups the records by date. The third statement walks through each date in order, looking up the value for each team on that date (using 0 if there is none).

You can test it like this:

foreach (var row in table)
{
    Console.WriteLine(row);
}

Output is:

{ Date = 1/1/2021 12:00:00 AM, Team = Team1, Value = 10 }
{ Date = 1/1/2021 12:00:00 AM, Team = Team2, Value = 10 }
{ Date = 1/1/2021 12:00:00 AM, Team = Team3, Value = 10 }
{ Date = 1/2/2021 12:00:00 AM, Team = Team1, Value = 20 }
{ Date = 1/2/2021 12:00:00 AM, Team = Team2, Value = 20 }
{ Date = 1/2/2021 12:00:00 AM, Team = Team3, Value = 0 }
{ Date = 1/3/2021 12:00:00 AM, Team = Team1, Value = 0 }
{ Date = 1/3/2021 12:00:00 AM, Team = Team2, Value = 0 }
{ Date = 1/3/2021 12:00:00 AM, Team = Team3, Value = 30 }