选择很多TOP 1

Question

在SQL Server 2008数据库中,我有以下表:

CREATE TABLE t_DeviceType
(
    ID INTEGER PRIMARY KEY IDENTITY, 
    Device VARCHAR(32) NOT NULL 
)

CREATE TABLE t_Device
(
    ID INTEGER PRIMARY KEY IDENTITY, 
    DeviceTypeID INTEGER NOT NULL,
    IPAddress INTEGER NOT NULL,
    RegTime DATETIME NOT NULL,
    IsActive BIT NOT NULL
)

现在我想要一个查询,它将为所有设备类型提取最后一个已注册但仍处于活动状态的设备(如果有).

对于单个给定设备,这很容易:

SELECT TOP 1 *
FROM t_Device
WHERE IsActive = 1
  AND DeviceTypeID = 42 -- For example
ORDER BY RegTime DESC

但是,我需要所有可能的价值DeviceTypeID.

我以为我可以用JOIN声明来做,但我找不到限制JOIN有效拉动的记录数量.

我尝试了以下方法:

SELECT t_DeviceType.ID AS TypeID, 
       t_DeviceType.Device, 
       Device.ID AS DeviceID, 
       Device.IPAddress AS IPAddress
FROM t_DeviceType
JOIN ( 
    SELECT TOP 1 t_Device.ID, DeviceTypeID, IPAddress 
    FROM t_Device
    JOIN t_DeviceType ON DeviceTypeID = t_DeviceType.ID
    WHERE IsActive = 1
    ORDER BY RegTime DESC
) AS Device ON Device.ConsoleTypeID = t_DeviceType.ID

但这只返回了任何类型的最后一个注册的devide.

有人有建议吗？

谢谢,

Answer 1

您希望获得设备列表及其关联的最大RegTime.

SELECT DeviceTypeID, max(RegTime) as LatestRegTime
FROM t_Device
WHERE IsActive = 1
GROUP BY DeviceTypeID

现在您可以将其用作派生表并加入它(而不是您尝试的选择顶部1):

SELECT t_DeviceType.ID AS TypeID, 
       t_DeviceType.Device, 
       Device.ID AS DeviceID, 
       Device.IPAddress AS IPAddress
FROM t_DeviceType
JOIN ( 
    SELECT t_Device.ID, 
           IPAddress, 
           DeviceTypeID, 
           max(RegTime) as LatestRegTime
    FROM t_Device
    WHERE IsActive = 1
    GROUP BY t_Device.ID, IPAddress, DeviceTypeID
) AS Device ON Device.DeviceTypeID = t_DeviceType.ID