数据量较大，求高效SQL或存储过程！
--设备表
CREATE TABLE t_Device
(
  DevId [int] PRIMARY KEY ,
  DevName [varchar](10) NOT NULL
)
 GO
 
--告警数据表
CREATE TABLE t_AlarmData
(
  DevId int NOT NULL,
  AlarmData int NOT NULL,
  AlarmTime datetime NOT NULL,
  CONSTRAINT PK_DA PRIMARY KEY (DevId,AlarmTime)
)
GO  

--实时数据表
CREATE TABLE t_RealData
(
  DevId int NOT NULL,
  RealData int NOT NULL,
  RealTime datetime NOT NULL,
  CONSTRAINT PK_DR PRIMARY KEY (DevId,RealTime)
)
GO  

--要求查出 每个设备最新告警时间的数据和与告警时间相等或之后的最大的实时数据

insert into t_Device (DevId,DevName) values(1,'设备A')
insert into t_Device (DevId,DevName) values(2,'设备B')
insert into t_Device (DevId,DevName) values(3,'设备C')


insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,6,'2012-6-12 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,10,'2012-6-11 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,9,'2012-6-10 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(2,14,'2012-6-11 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(2,5,'2012-6-10 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(3,9,'2012-6-11 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(3,12,'2012-6-10 00:00:00')

insert into t_RealData (DevId,RealData,RealTime) values(1,6,'2012-6-12 11:00:00') --需要显示
insert into t_RealData (DevId,RealData,RealTime) values(1,10,'2012-6-12 10:00:00')
insert into t_RealData (DevId,RealData,RealTime) values(1,9,'2012-6-10 9:00:00')
insert into t_RealData (DevId,RealData,RealTime) values(2,19,'2012-6-11 10:00:00') --需要显示
insert into t_RealData (DevId,RealData,RealTime) values(2,5,'2012-6-10 8:00:00')
insert into t_RealData (DevId,RealData,RealTime) values(3,9,'2012-6-9 00:00:00')
insert into t_RealData (DevId,RealData,RealTime) values(3,12,'2012-6-9 00:00:00')


结果应该是：

DevId DevName AlarmData AlarmTime RealData RealTime
 1 设备A 6 2012-6-12 6 2012-6-12 11:00:00
 2 设备B 14 2012-6-11 19 2012-6-11 10:00:00  
 3 设备C 9 2012-6-11 null null

------解决方案--------------------

SQL code

--设备表
CREATE TABLE t_Device
(
  DevId [int] PRIMARY KEY ,
  DevName [varchar](10) NOT NULL
)
 GO
--告警数据表
CREATE TABLE t_AlarmData
(
  DevId int NOT NULL,
  AlarmData int NOT NULL,
  AlarmTime datetime NOT NULL,
  CONSTRAINT PK_DA PRIMARY KEY (DevId,AlarmTime)
)
GO   

--实时数据表
CREATE TABLE t_RealData
(
  DevId int NOT NULL,
  RealData int NOT NULL,
  RealTime datetime NOT NULL,
  CONSTRAINT PK_DR PRIMARY KEY (DevId,RealTime)
)
GO   

--要求查出 每个设备最新告警时间的数据和与告警时间相等或之后的最大的实时数据

insert into t_Device (DevId,DevName) values(1,'设备A')
insert into t_Device (DevId,DevName) values(2,'设备B')
insert into t_Device (DevId,DevName) values(3,'设备C')


insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,6,'2012-6-12 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,10,'2012-6-11 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(1,9,'2012-6-10 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(2,14,'2012-6-11 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(2,5,'2012-6-10 00:00:00')
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(3,9,'2012-6-11 00:00:00') --最新告警
insert into t_AlarmData (DevId,AlarmData,AlarmTime) values(3,12,'2012-6-10 00:00:00')

insert into t_RealData (DevId,RealData,RealTime) values(1,6,'2012-6-12 11:00:00') --需要显示
insert into t_RealD