关于变量赋值的问题~~急

[常飞梦]

我想在一个表里求一个字段的最大值，想用执行字符串的方法实现，可是我用以下的方法写怎么也求不出，得出是空值。请问谁给分析一下~~

declare @str1 varchar(500)
declare @V_servicetable varchar(500)
declare @v_currentid numeric

set @V_servicetable='tb'

set @str1='select ' + cast(@v_currentid as varchar) + ' = max(id) from ' + @V_servicetable
print @v_currentid
exec (@str1)

[九斤半]

参考：

declare @str nvarchar(4000)
declare @i int
set @str = 'select @i = count(*) from sysobjects'
exec sp_executesql @str,N'@i int out',@i out
select @i

[冷箫轻笛]

declare @str1 varchar(500)
declare @V_servicetable varchar(500)
set @str1 = ''
set @V_servicetable='tb'

set @str1 = '
declare @v_currentid numeric
select @v_currentid = max(id) from '+ @V_servicetable + '
print @v_currentid '

exec (@str1)

[常飞梦]

我就是想用SQL的字符串给变量赋值，楼上的变量只能在SQL里能用，在外面就不能用了，我想在下面的语句还要调用这个变量！！

[常飞梦]

还有我想在后面加一个变量条件应该怎么加？
比如：
set @str1='select ' + cast(@v_currentid as varchar) + ' = max(id) from ' + @V_servicetable+ ' where id = '+ @v_currentid + ';'

可这样好像也有毛病！！

[hui_hui_2007]

还有我想在后面加一个变量条件应该怎么加？
比如：
set @str1='select ' + cast(@v_currentid as varchar) + ' = max(id) from ' + @V_servicetable+ ' where id = '+ @v_currentid + ';'

可这样好像也有毛病！！
-------------------------
cast(@v_currentid as varchar) + ' = max(id)
有问题吧,为何这样写

[OracleRoob]

--如何将exec执行结果放入变量中？

declare @v_currentid int, @sqls nvarchar(4000) ,@tb varchar(100)
set @tb='hb168_message30'

set @sqls='select @a=max(id) from ' + @tb
exec sp_executesql @sqls,N'@a int output',@v_currentid output
select @v_currentid

[hui_hui_2007]

我写了个示例,你看一下吧,呵
create table #
(
myid int
)

insert into #
select 1000 union all
select 2 union all
select 3 union all
select 23 union all
select 14 union all
select 26 union all
select 71 union all
select 29 union all
select 51 union all
select 392

select * from #

go

declare @str varchar(100)
set @str='declare @maxid int select @maxid=max(myid) from # print @maxid'
exec (@str)

[hui_hui_2007]

又一种写法:
create table #
(
myid int
)

insert into #
select 1000 union all
select 2 union all
select 3 union all
select 23 union all
select 14 union all
select 26 union all
select 71 union all
select 29 union all
select 51 union all
select 392

select * from #

go

declare @str nvarchar(4000)
declare @maxid int
set @str = N'select @maxid = max(myid) from #'
exec sp_executesql @str,N'@maxid int out',@maxid out
select @maxid