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