Question

我有一个SQL Server 2008表，其中包含Designationid列和NewDesignationid列。我想根据日期找到加入的指定ID。我不知道怎么做。

我的SQL Server表结构如下：

 Designationid  NewDesignationID    UniqueID    EmployeeID  FromDate    ToDate  Comments    ApprovedDate    ModifiedBy  ModifiedDate    StatusID    ChangeTo    CreatedBy   CreatedDate
12  25  258 1700    7/19/2011   7/27/2011   Designation Change  7/28/2011   61  7/28/2011   8   Executive - HR - Associate Executive -  HR  61  7/28/2011
25  132 458 1700    7/28/2011   6/27/2012   Designation Change  6/28/2012   1700    6/28/2012   8   Associate Executive -  HR To RMG Executive  1700    6/28/2012
25  132 510 1700    7/13/2012   8/2/2012    Designation Change  8/3/2012    1700    8/3/2012    8   Associate Executive -  HR To RMG Executive  1700    8/3/2012
132 25  507 1700    6/28/2012   7/12/2012   Designation Change  7/13/2012   1896    7/13/2012   8   RMG Executive To Associate Executive -  HR  1896    7/13/2012

我也想为每个特定的EmployeeId做这件事。目前我已经为一个EmployeeId。

这样做了

Answer 1

如果您只需要一行，则可以使用top 1和order by ...：

select top 1
    Designationid, NewDesignationID
from <Table>
order by FromDate asc

如果您希望每位员工使用此功能，则可以使用row_number()功能：

;with cte as (
     select *, row_number() over(partition by EmployeeID order by FromDate asc) as rn
)
select
    EmployeeID, Designationid, NewDesignationID
from cte
where rn = 1

你的问题中有一些微妙之处，而且这个问题在很多时候都有过讨论。其中一个问题是 - 如果你有2条具有相同FromDate和相同EmployeeID的记录，你想要获得两个记录，还是只想一个（如果你想要一个，那么你想要哪一个？）。如果你想要两个行，你可以使用这样的方法：

select
    T.EmployeeID, T.Designationid, T.NewDesignationID
from <Table> as T
where
    not exists (
        select *
        from <Table> as TT
        where TT.EmployeeID = T.EmployeeID and TT.FromDate < T.FromDate
    )

或

select
    T.EmployeeID, T.Designationid, T.NewDesignationID
from <Table> as T
where
    exists (
        select 1
        from <Table> as TT
        group by TT.EmployeeID
        having min(TT.FromDate) = T.FromDate
    )

如果您只想要一个，可以使用row_number()（见上文）或outer apply：

select
    T.EmployeeID, TT.Designationid, TT.NewDesignationID
from (select distinct TT.EmployeeID from <Table> as TT) as T
    outer apply (
        select top 1
            TT.Designationid, TT.NewDesignationID
        from <Table> as TT
        where TT.EmployeeID = T.EmployeeID
        order by TT.FromDate asc
    ) as TT

Answer 2

尝试这种结构（你的样本不清楚）

SELECT EmployeeID,
      min(FromDate)
FROM [EmployeeTable]
GROUP BY EmployeeID

从表中查找最小值的日期值

2 个答案: