原来，union的优先级很高。比如
select stdname,[subject]='化学',化学 as score from #student2 union all select stdname,[subject]='数学',数学 as score from #student2 union all select stdname,[subject]='物理',物理 as score from #student2 union all select stdname,[subject]='语文',语文 as score from #student2 order by stdname,[subject]
等同于
select * from( select stdname,[subject]='化学',化学 as score from #student2 union all select stdname,[subject]='数学',数学 as score from #student2 union all select stdname,[subject]='物理',物理 as score from #student2 union all select stdname,[subject]='语文',语文 as score from #student2 ) as a order by stdname,[subject]