如何根据其他列数据过滤掉相似的行（在某些列上相等）

Question

我如何选择名字和姓氏上不同的所有行，并且在有重复的地方选择具有较高SomeDate的行，然后选择Id 如果仍然重复，例如

：

| Id | Forename | Surname | SomeDate   |
----------------------------------------
| 1  | Bill     | Power   | 2011-01-01 |
| 2  | James    | Joyce   | 2011-02-01 |
| 3  | Peter    | Lennon  | 2011-03-01 |
| 4  | John     | Sellers | 2011-04-01 |
| 5  | James    | Joyce   | 2011-05-01 |
| 6  | Peter    | Lennon  | 2011-03-01 |

结果：

| Id | Forename | Surname | SomeDate   |
----------------------------------------
| 1  | Bill     | Power   | 2011-01-01 |
| 4  | John     | Sellers | 2011-04-01 |
| 5  | James    | Joyce   | 2011-05-01 |
| 6  | Peter    | Lennon  | 2011-03-01 |

我如何

1. 在T-SQL从 DataTable 中实现此目的
2. 使用 C#

Answer 1

假设 SQL Server 2005+，使用：

SELECT x.id,
       x.forename,
       x.surname,
       x.somedate
  FROM (SELECT t.id,
               t.forename,
               t.surname,
               t.somedate,
               ROW_NUMBER() OVER (PARTITION BY t.forename, t.surname 
                                      ORDER BY t.somedate DESC, t.id DESC) AS rank
          FROM TABLE t_ x
WHERE x.rank = 1

一个有风险的方法是：

  SELECT MAX(t.id) AS id,
         t.forename,
         t.surname,
         MAX(t.somedate) AS somedate
    FROM TABLE t
GROUP BY t.forename, t.surname

Answer 2

我倾向于对非分组值使用子选择。

SELECT Forename, Surname, 
    (SELECT TOP 1 Id FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
     ORDER BY m.SomeDate DESC) AS Id
    (SELECT TOP 1 SomeDate FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
     ORDER BY m.SomeDate DESC) AS SomeDate
FROM myTable m
GROUP BY Forename, Surname

或者您可以在 WHERE 子句中对其进行过滤：

SELECT Id, Forename, Surname, SomeDate
FROM myTable m
WHERE m.Id = (SELECT TOP 1 Id FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
    ORDER BY m.SomeDate DESC)

恐怕这两种方法都不会非常有效，但如果需要的话，索引调整会改善这种情况。

对于数据表示例，您基本上会做同样的事情。

var recs = from record in dataTable
           where record.Id == 
               (from rec in dataTable
                where rec.Forename == record.Forename && rec.Surname == record.Surname
                orderby rec.SomeDate descending
                select rec.Id).First()
           select record;