来自多个列和表的linq最大日期

Question

Objective：table1、table2、table3 每个日期列都有多个不同名称的日期列。

我需要为相应的 Id 拉回这些列的最大日期；最后修改日期。

我可以构建一个数组形式的 let 变量，然后选择最大值吗？

或者我需要在查询中做一些 case 语句吗？

我所做的每一次尝试都失败了，寻求建议....

var query = from t1 in table1
            join t2 in table2
                on t1.Id equals t2.Id
            join t3 in table3
                on t1.Id equals t3.Id
            select new 
            {
                 Id = t1.Id
                 LastModified = <max date from multiple columns from different tables>
            };

Answer 1

您可以将表转换为通用类型，然后取最大日期。

var list1 = new List<Table1>();
var list2 = new List<Table2>();
var list3 = new List<Table3>();

var lastRecord = list1.Select(x => new {Id = x.Id, DateTime = x.Dt})
            .Union(list2.Select(x => new {Id = x.Id2, DateTime = x.Date}))
            .Union(list3.Select(x => new {Id = x.Id3, DateTime = x.DateDiffName}))
            .OrderByDescending(t => t.DateTime)
            .First();

Answer 2

使用@Alexander 的方法，并将您的查询切换到 Lambda/Fluent LINQ 语法，因为 Union 在查询语法中不（直接）可用。

首先，从要比较的每个 table 中取出所有日期列，并将它们展平成单独的行：

var t1Dates = table1.Select(t1 => new { t1.Id, DT = t1.dc1 })
                    .Union(table1.Select(t1 => new { t1.Id, DT = t1.dc2 }));
var t2Dates = table2.Select(t2 => new { t2.Id, DT = t2.t2dc1 })
                    .Union(table2.Select(t2 => new { t2.Id, DT = t2.t2dc2 }));
var t3Dates = table3.Select(t3 => new { t3.Id, DT = t3.t3dc1 })
                    .Union(table3.Select(t3 => new { t3.Id, DT = t3.t3dc2 }));

然后，像最初那样加入单独的日期：

var t123Dates = t1Dates.Join(t2Dates, t1 => t1.Id, t2 => t2.Id, (t1, t2) => new { t1.Id, DT1 = t1.DT, DT2 = t2.DT })
                       .Join(t3Dates, t12 => t12.Id, t3 => t3.Id, (t12, t3) => new { t12.Id, t12.DT1, t12.DT2, DT3 = t3.DT });

然后，将连接的值展平到不同的行中：

var tAllDates = t123Dates.Select(t123 => new { t123.Id, DT = t123.DT1 })
                         .Union(t123Dates.Select(t123 => new { t123.Id, DT = t123.DT2 }))
                         .Union(t123Dates.Select(t123 => new { t123.Id, DT = t123.DT3 }));

最后，您可以按 Id 对扁平化结果进行分组，并找到每个 Id:

对应的最新日期

var query = tAllDates.GroupBy(ta => ta.Id)
                     .Select(tag => new {
                         Id = tag.Key,
                         LastModified = tag.OrderByDescending(ta => ta.DT).First().DT
                     });

Answer 3

如果您只对 Id 和 LastModified Date 感兴趣，我的建议是使用 Select 到 select Id 和Date，然后 Orderby 降序日期并取第一个：

var result = dbContext.Table1.Select(row => new
{
    Id = row.Id,
    Date = row.Date,
})
.Concat(dbContext.Table2.Select(row => new
{
    Id = row.Id,
    Date = row.Date,
})
.Concat(dbContext.Table3.Select(row => new
{
    Id = row.Id,
    Date = row.Date,
})
.OrderByDescending(row => row.Date)
.FirstOrDefault();

简而言之：从表 1 的每一行中获取 ID 和日期。对 table2 和 table3 中的行执行类似的操作。将这些 selected 项一个接一个地放置，按数据降序排列，取第一个，如果根本没有行，则为 null。

简单的comme bonjour！

Answer 4

根据我的理解，我创建了以下代码。请看一看。代码本身包含注释：

 static void LinkDate()
    {
        DataTable table1 = new DataTable();
        table1.Columns.Add("Id");
        table1.Columns.Add("Date", typeof(System.DateTime));

        DataTable table2 = new DataTable();
        table2.Columns.Add("Id");
        table2.Columns.Add("Date", typeof(System.DateTime));

        DataTable table3 = new DataTable();
        table3.Columns.Add("Id");
        table3.Columns.Add("Date", typeof(System.DateTime));

        DataRow row = table1.NewRow();
        row[0] = "1";
        row[1] = Convert.ToDateTime("01/22/2020");
        table1.Rows.Add(row);
        row = table1.NewRow();
        row[0] = "2";
        row[1] = Convert.ToDateTime("01/23/2020");
        table1.Rows.Add(row);
        row = table1.NewRow();
        row[0] = "3";
        row[1] = Convert.ToDateTime("02/22/2020");
        table1.Rows.Add(row);
        row = table1.NewRow();
        row[0] = "4";
        row[1] = Convert.ToDateTime("03/22/2020");
        table1.Rows.Add(row);
        row = table1.NewRow();
        row[0] = "5";
        row[1] = Convert.ToDateTime("05/22/2020");
        table1.Rows.Add(row);


        row = table2.NewRow();
        row[0] = "1";
        row[1] = Convert.ToDateTime("01/25/2020");
        table2.Rows.Add(row);
        row = table2.NewRow();
        row[0] = "2";
        row[1] = Convert.ToDateTime("04/21/2020");
        table2.Rows.Add(row);
        row = table2.NewRow();
        row[0] = "3";
        row[1] = Convert.ToDateTime("02/11/2020");
        table2.Rows.Add(row);
        row = table2.NewRow();
        row[0] = "8";
        row[1] = Convert.ToDateTime("08/12/2020");
        table2.Rows.Add(row);
        row = table2.NewRow();
        row[0] = "9";
        row[1] = Convert.ToDateTime("02/25/2020");
        table2.Rows.Add(row);

        row = table3.NewRow();
        row[0] = "1";
        row[1] = Convert.ToDateTime("03/20/2020");
        table3.Rows.Add(row);
        row = table3.NewRow();
        row[0] = "2";
        row[1] = Convert.ToDateTime("01/11/2020");
        table3.Rows.Add(row);
        row = table3.NewRow();
        row[0] = "12";
        row[1] = Convert.ToDateTime("01/21/2020");
        table3.Rows.Add(row);
        row = table3.NewRow();
        row[0] = "8";
        row[1] = Convert.ToDateTime("09/22/2020");
        table3.Rows.Add(row);
        row = table3.NewRow();
        row[0] = "11";
        row[1] = Convert.ToDateTime("01/20/2020");
        table3.Rows.Add(row);


        var query = from t1 in table1.AsEnumerable()
                    join t2 in table2.AsEnumerable()
                        on t1.Field<string>("Id") equals t2.Field<string>("Id")
                    join t3 in table3.AsEnumerable()
                        on t1.Field<string>("Id") equals t3.Field<string>("Id")
                    select new
                    {
                        Id = t1.Field<string>("Id"),
                        LastModified = Max(t1.Field<DateTime>("Date"), t2.Field<DateTime>("Date"), t3.Field<DateTime>("Date"))
                    };

    }
    static DateTime Max(DateTime a, DateTime b, DateTime c)
    {
        //Get max value between three.
        DateTime[] aa = { a, b, c };
        return aa.Max();
    }

来自多个列和表的linq最大日期

linq max date from multiple columns and tables

c#

linq

entity-framework-6