Linq Lambda 表达式 - 左连接

Question

1. 我有 table1 和一组标准值 monthname 和 monthnumber。

2. 另一个table2 有很多列，但我想与列名CreateDate.Month 进行比较。

3. 想获取table2的所有记录条件是（一月到四月的数据）

4. 在表 2 中，如果相应月份的记录不可用，则不可用月份的计数应为 0

我正在寻找作为 Linq 语句的答案。

        List<monthNames> defaultMonthNames = new List<monthNames>
        {
            new monthNames { monthNumber = 1, monthName = "Jan"},
            new monthNames { monthNumber = 2, monthName = "Feb"},
            new monthNames { monthNumber = 3, monthName = "Mar"},
            new monthNames { monthNumber = 4, monthName = "Apr"},
            new monthNames { monthNumber = 5, monthName = "May"},
            new monthNames { monthNumber = 6, monthName = "Jun"},
            new monthNames { monthNumber = 7, monthName = "Jul"},
            new monthNames { monthNumber = 8, monthName = "Aug"},
            new monthNames { monthNumber = 9, monthName = "Sep"},
            new monthNames { monthNumber =10, monthName = "Oct"},
            new monthNames { monthNumber = 11, monthName = "Nov"},
            new monthNames { monthNumber = 12, monthName = "Dec"},

        };
        var Toprecords = somedata.Join(defaultMonthNames,
                        tc => tc.CreatedDate.Month, dmn => 
                        dmn.monthNumber,
                       (tc, dmn) => new 
                        { TopClients = tc.CreatedDate.Month, 
                          defaultMonthNames = dmn 
                        }).GroupBy(a => new
                        {
                             a.ClientId,
                             a.CreatedDate.Month
                        }).Select(m => new
                        {
                            label = m.Select(a => 
                            a.ClientName).FirstOrDefault(),
                            NoOfOrders = m.Count(),
                            monthnumber =                           
                    m.Select(a=>a.CreatedDate.Month).FirstOrDefault()
                     }).ToList();

表1：

EmpName createdDate
Name1    12-01-2019
Name2    02-02-2019
Name3    05-02-2019
Name4    08-02-2019
Name5    02-03-2019
Name6    04-09-2019

表2：

monthName monthnumber
Jan         1
Feb         2
Mar         3
Apr         4

预期输出：

EmpName  MonthName  count
Name1    Jan          1
Name1    Feb          0
Name1    Mar          0
Name1    Apr          0
Name2    Jan          0
Name2    Feb          1
Name2    Mar          0 

Answer 1

您尝试做的事情非常繁琐，我认为在这种情况下，混合使用 LINQ 查询和 lambda 语法可能会更清晰。作为一个起点，下面的代码应该可以得到你想要的结果，所以我希望它对你有所帮助！

var someData = new List<SomeData>
{
    new SomeData {EmpName = "Name 1", CreatedDate = new DateTime(2019, 1, 12)},
    new SomeData {EmpName = "Name 2", CreatedDate = new DateTime(2019, 2, 2)},
    new SomeData {EmpName = "Name 3", CreatedDate = new DateTime(2019, 2, 5)},
    new SomeData {EmpName = "Name 4", CreatedDate = new DateTime(2019, 2, 8)},
    new SomeData {EmpName = "Name 5", CreatedDate = new DateTime(2019, 3, 2)},
    new SomeData {EmpName = "Name 6", CreatedDate = new DateTime(2019, 4, 9)},
};

var defaultMonthNames = new List<MonthNames>
{
    new MonthNames {MonthName = "Jan", MonthNum = 1},
    new MonthNames {MonthName = "Feb", MonthNum = 2},
    new MonthNames {MonthName = "Mar", MonthNum = 3},
    new MonthNames {MonthName = "Apr", MonthNum = 4},
    new MonthNames {MonthName = "May", MonthNum = 5},
    new MonthNames {MonthName = "Jun", MonthNum = 6},
    new MonthNames {MonthName = "Jul", MonthNum = 7},
    new MonthNames {MonthName = "Aug", MonthNum = 8},
    new MonthNames {MonthName = "Sep", MonthNum = 9},
    new MonthNames {MonthName = "Oct", MonthNum = 10},
    new MonthNames {MonthName = "Nov", MonthNum = 11},
    new MonthNames {MonthName = "Dec", MonthNum = 12},
};

var crossJoined = from sd in someData
                    from dmn in defaultMonthNames.Where(dmn => dmn.MonthNum <= 4)
                    select new
                    {
                        sd.EmpName,
                        dmn.MonthName,
                        dmn.MonthNum
                    };

var dataMonthCounts = someData
    .GroupBy(sd => new { sd.EmpName, sd.CreatedDate.Month })
    .Select(group => new
    {
        EmpName = group.Key.EmpName,
        MonthNum = group.Key.Month,
        Count = group.Count()
    });

var results = from cj in crossJoined
                join dmc in dataMonthCounts
                    on new { cj.EmpName, cj.MonthNum } equals new { dmc.EmpName, dmc.MonthNum } into combined
                from c in combined.DefaultIfEmpty()
                select new
                {
                    cj.EmpName,
                    cj.MonthName,
                    Count = c?.Count ?? 0
                };