sql错误

[wishY]

declare @adnum int 
      
        Declare @sql_adnum2 nvarchar(500)
      set @sql_adnum2 = 'SELECT @adnum = COUNT(AdminInfo.AdminID)
	  FROM AdminInfo INNER JOIN
      AdminGrade ON AdminInfo.AdminGradeID = AdminGrade.AdminGradeID INNER JOIN
      Sub_department ON 
      AdminInfo.Sub_departmentID = Sub_department.Sub_departmentID
      where AdminInfo.Active = ''true'' and Sub_department.Sub_departmentID = '''+cast(@Sub_departmentID as nvarchar(4))+'''
      '
      	  exec (@sql_adnum2)
	  select @adnum 

这样为什么查不出来，要怎么修改？错误：@adnum未定义.