Question

如何选择 Forename 和 Surname 上的所有不同行，如果有重复，请选择 SomeDate 更高的行，然后< strong> Id 如果仍然重复，例如

有关：

| 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 |

我怎样才能在

中实现这一目标

T-SQL
使用C＃从DataTable

Answer 1

假设SQL Server 2005+，请使用：

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

风险方法是：

  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

我倾向于对未分组的值使用子选择。

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

或者你可以在WHERE子句中过滤它：

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)

我担心两者都不会非常高效，但如果需要的话，指数调整会改善它。

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;

如何根据其他列数据过滤掉类似的行（在某些列上相等）

2 个答案: