1:普通SQL语句可以用Exec执行
eg:   Select * from tableName
      Exec(\'select * from tableName\')
      sp_executesql N\'select * from tableName\'    -- 请注意字符串前一定要加N

2:字段名，表名，数据库名之类作为变量时，必须用动态SQL
eg:  
declare @fname varchar(20)
set @fname = \'[name]\'
Select @fname from sysobjects                     -- 错误
Exec(\'select \' + @fname + \' from sysobjects\')     -- 请注意 加号前后的 单引号的边上要加空格
exec sp_executesql N\' select \' + @fname + \' from sysobjects\'
当然将字符串改成变量的形式也可
    declare @s varchar(1000)
    set @s = \'select \' + @fname + \' from sysobjects\'
    Exec(@s)                -- 成功
    exec sp_executesql @s   -- 此句会报错

    declare @s Nvarchar(1000)  -- 注意此处改为nvarchar(1000)
    set @s = \'select \' + @fname + \' from sysobjects\'
    Exec(@s)                -- 成功   
    exec sp_executesql @s   -- 此句正确，

3: 输出参数
eg:
declare @num,
        @sqls
set @sqls=\'select count(*) from  \' + @servername + \'.a.dbo.b\'
exec(@sqls)
我如何能将exec执行的结果存入变量@num中

declare @num int,
        @sqls nvarchar(4000)
set @sqls=\'select @a=count(*) from \'+@servername+\'.a.dbo.b\'
exec sp_executesql @sqls,N\'@a int output\',@num output
select @num