如何在 Linq 中将行转换为列

Question

我的表 ReportData 有以下记录

Agency	Year	Month	MonthCount
One	2017	Jan	4
One	2017	Feb	6
One	2017	Nov	29
One	2017	Dec	38
One	2018	Jan	20
One	2019	Feb	18
Two	2017	Jan	24
Two	2017	Feb	66
Three	2017	Nov	9
Four	2017	Dec	8
Four	2018	Jan	22
Four	2019	Feb	48

如何转换成下表？请让我知道 Linq 声明。提前感谢您的帮助。

Agency	Month	2017	2018	2019
One	Jan	4	20	0
One	Feb	6	0	18
One	Nov	29	0	0
One	Dec	38	0	0
Two	Jan	24	0	0
Two	Feb	66	0	0
Three	Nov	9	0	0
Four	Jan	0	22	0
Four	Feb	0	0	48
Four	Dec	8	0	0

源类结构

    public class ReportingData
{
    public string AgencyName { get; set; }
    public int Year { get; set; }
    public string MonthNumnber { get; set; }
    public string Month { get; set; }
    public int MonthCount { get; set; }
}

Answer 1

在我看来，每个 [Agency, Month] 组合，您需要 2017..2019 年的 MonthCounts。

我想知道你几年后想要什么：只有过去三年？还是 2017 年.. 2025 年？但这是后来的问题。

我的建议是使 ReportData 组具有相同的 [Agency, Month] 组合值。使用 overload of Enumerable.GroupBy that has a parameter resultSelector 精确定义结果。如果您的数据在不同的进程（通常是数据库）中，请使用 IQueryable 版本。

IEnumerable<Report> reportData = ...

// make groups with same values for [Agency, Month] combination
var result = reportData.GroupBy(report => new
{
    Agency = report.Agency,
    Month = report.Month,
},

// parameter resultSelector: for every [Agency, Month] combination,
// and all reports that have this [Agency, Month] combination,
// make one new:
(agencyMonthCombination, reports] => new
{
    Agency = agencyMonthCombination.Agency,
    Month = agencyMonthCombination.Month,

    Year2017 = reports.Where(report => report.Year == 2017)
                      .Select(report => report.MonthCount)
                      .Sum();

    Year2018 = reports.Where(report => report.Year == 2018)
                      .Select(report => report.MonthCount)
                      .Sum();

    Year2019 = reports.Where(report => report.Year == 2018)
                      .Select(report => report.MonthCount)
                      .Sum();

如果您希望每年只有一份报告，则不必求和，您可以使用 FirstOrDefault。

您看到了这种命名年份 2017..2019 的方法的缺点。如果您在所有年份或过去 N 年中编写代码会容易得多：

// parameter resultSelector
(agencyMonthCombination, reports] => new
{
    Agency = agencyMonthCombination.Agency,
    Month = agencyMonthCombination.Month,

    // group the reports with this [Agency, Month] combination in years
    MonthCounts = reports.GroupBy(report => report.Year,

        (year, reportsInThisYear) => new
        {
            Year = year,
            MonthCount = reportsInThisYear.Select(report => report.MonthCount).Sum(),
        })

        // if you only want the last N: orderby descending Year and Take(N)
        .OrderByDescending(yearMonthCount => yearMonthCount.Year)
        .Take(N)

        .ToList(),
    });